Annoying Coworkers don't go away, they just get replaced!- the fat lady sings,p4

[Shopaholic]

If what you want is for him to say, "I'm sorry, I shouldn't have put you at risk without your knowledge, and I should have asked you before touching the apparatus" then go talk to him or his mentor and say that. That's a reasonable request. What's not reasonable is throwing out a PA comment and hoping to get that back.

That isn't what I expect him to say.
I expect him to show some inclination that he understood what he did was wrong, if not by actually stating it (it might be too much to ask) then by body language.
His body language showed no sense of "I did wrong."

Yesterday he came in early, didn't say a word to anyone and set up an experiment. After an hour he noticed his experiment doesn't work. Who does he call? Me.
"Shopaholic, why won't my experiment work?"
"I don't know."
"The equipment is giving me an error message. What does it mean?"
"I don't know."
"You don't know??"
"No, look it up in the manual."
Apparently in Barney's world an electrical circuit doesn't need to be closed for there to be a current running...

I then chewed him out for leaving a sensitive piece of equipment where it could be easily damaged.

Today he insists on working on my bench despite the fact that a bench has been allocated for him to share with another student.
I warned him that I was working on something sensitive, and actually had to put a masking tape line down the bench to mark where he shouldn't cross. I feel like in grade school.
Next time I'm going to tell him that he ruined my experiment and there is no way he is getting near my stuff again, I was just too angry today for it to come out in an effective manner.

[Mental Magpie]

Honestly, let that original one go, and here's why.  You obviously already don't like him, and I have a feeling he's going to start picking up on that.  Every time he does something wrong that can't just be overlooked, tell him, but do so calmly.  Let that first one go, it's too late (simply because he's made a myriad of other mistakes since then), and focus on what he's doing now.  Allocated to another bench to share?  Remind him.  Leaving out sensitive equipment?  "You know not to do that."  Keep your voice calm, level, but firm.  He'll eventually get it (hopefully).

[Judah]

Today when I was working in Apparatus, I told Barney that I had a short break, but that I am still working in it. He didn't get it, just said nonchalantly that he didn't need it.

Today he insists on working on my bench despite the fact that a bench has been allocated for him to share with another student.
I warned him that I was working on something sensitive, and actually had to put a masking tape line down the bench to mark where he shouldn't cross. I feel like in grade school.
Next time I'm going to tell him that he ruined my experiment and there is no way he is getting near my stuff again, I was just too angry today for it to come out in an effective manner.

What I'm getting from your posts is that for some reason you seem to dance around an issue without directly addressing it, then get upset when Barney doesn't respond the way you want him to. In the first quote, I would have said, "I'm taking a break. That doesn't mean you can use the apparatus' it's still unavailable to you. This is just a break."

In the second quote you "warned him that I was working on something sensitive", but you didn't say, "You need to use the bench that was allocated for your use. You may not use my bench to work on as I am working on something sensitive."

Please be direct, it's the only way Barney's going to understand what you want.

[Shoo]

Today when I was working in Apparatus, I told Barney that I had a short break, but that I am still working in it. He didn't get it, just said nonchalantly that he didn't need it.

Today he insists on working on my bench despite the fact that a bench has been allocated for him to share with another student.
I warned him that I was working on something sensitive, and actually had to put a masking tape line down the bench to mark where he shouldn't cross. I feel like in grade school.
Next time I'm going to tell him that he ruined my experiment and there is no way he is getting near my stuff again, I was just too angry today for it to come out in an effective manner.

What I'm getting from your posts is that for some reason you seem to dance around an issue without directly addressing it, then get upset when Barney doesn't respond the way you want him to. In the first quote, I would have said, "I'm taking a break. That doesn't mean you can use the apparatus' it's still unavailable to you. This is just a break."

In the second quote you "warned him that I was working on something sensitive", but you didn't say, "You need to use the bench that was allocated for your use. You may not use my bench to work on as I am working on something sensitive."

Please be direct, it's the only way Barney's going to understand what you want.

I agree with this.

Just TELL him.  Don't hint at it.

[Shopaholic]

^You are both right. It is time that I let go of the original incident.

Update:
I had a talk with my PI today about work. My experiment from last week failed. He asked me why I thought that was, and I listed a number of reasons, among them I said there was an unfortunate incident when someone took over Apparatus, and due to the hazard extra cleaning was necessary and the experiment was delayed. I didn't name names, but I did say that was the reason I asked him to refresh the regulations during the group meeting.

PI told me that I have complete authority to throw something at someone when something like that happens, and acknowledged also the hazard. He gave a cute armed forces analogy about me being the commander of my project and having the authority to put someone in his/her place when I see they are jeopardizing the advancement of my research.

I told PI I looked for the most efficient way to get the point accross, but that next time I will indeed throw something.
PI replied "...and call me to throw things with you when that happens."

One of my favorite things about my PI is that he always has my back .

[Marguette]

I assume you meant it light-heartedly, but throwing something is just hinting around too. If Annoying Coworker is clueless enough to mess in your space and your equipment, throwing something will tell him you’re upset and annoyed, but it still won’t tell him what he’s doing wrong. Like Shoo said, I encourage you to use your words. And not just to tell him what not to do, but tell him what to do (the “herding toddlers” principle). I.e., not just “don’t work on my bench,” but “that bench over there is your bench. You need to work there, not here.”

[Deetee]

In the second quote you "warned him that I was working on something sensitive", but you didn't say, "You need to use the bench that was allocated for your use. You may not use my bench to work on as I am working on something sensitive."

Please be direct, it's the only way Barney's going to understand what you want.

I agree with this.

Just TELL him.  Don't hint at it.

I also agree.
He is being annoying and dangerous and going against lab protocols (I worked in a lab for years and years-the guy is a menace)

But WHY let him stay on your bench? In my lab, bench space (and equipment) was inviolate. If some undergrad or new student had started using my space, I would have told them that he could not. I would have done it nicely-I would have asked if they were assigned a space and then helped them reconfigure the space or asked their supervising student to help with any deficiencies, but they do NOT get my space.

He isn't getting it because you aren't telling him (AND he is dense) Some people require more directness than others. He is one of them.

I did work with one guy who was pushy and a bit dense (super smart but clueless). However, I just decided he had Aspergers and was much more direct, almost rude, to him and we started to get along great after that because I didn't expect him to understand things unless I was blunt and direct (and once I was, he was very considerate).

[DavidH]

You said he insists on working on your bench.  Can we define "insists"? 

Did you ever say to him, not hint, but actually say, "This is my bench, the one assigned to you is over there.  You need to conduct your experiment there, not here?"

What is your role in the lab compared to his?  Are you his supervisor, mentor, colleague? 

The way to get what you want is to tell him, not hint, not put tape on your bench, not chew him out, just tell him what he should be doing. 

[O'Dell]

Whatever, I told him to get his stuff out NOW because Apparatus needed to be sterilized and I needed to use it in 10 minutes. He said OK, but continued working for another 10 minutes.

How much more explicit does the OP need to be? 

[DavidH]

Since you asked, here are some examples:

OP:  I told Barney that I had a short break, but that I am still working in it.
More explicit:  I'm still angry about yesterday, do you now understand what you did wrong and what the correct procedures are?

OP: I warned him that I was working on something sensitive, and actually had to put a masking tape line down the bench to mark where he shouldn't cross.
More explicit:  This is my bench, the one assigned to you is over there.  Please don't use my bench and use the one assigned to you instead.

[Deetee]

Whatever, I told him to get his stuff out NOW because Apparatus needed to be sterilized and I needed to use it in 10 minutes. He said OK, but continued working for another 10 minutes.

How much more explicit does the OP need to be? 

"I don't think you understand. I need to run my experiment RIGHT NOW. Get your stuff out of the way now before I need to throw it all away myself."

[Shopaholic]

I read all of your posts. I do have a problem with being direct because I very often come off abrasive. I'm not good at confrontation.
I am the PhD student who has been in the lab longest. Maddie and I share a bench, and we get along well.

This morning I told Barney I needed to talk to him, and in a calm, quiet voice told him the following:
"Please take your things and move to other bench. I'm asking you to do this for two reasons: one, the bench is becoming very crowded and two, I am not at ease working next to you.
I am still angry about what happened last week. Your behaviour was unprofessional, irresponsible and frankly, rude..."

At this point Barney interjected with "But I apologized for that!"

I continued, in the same calm, quiet voice:
"Actually, no you didn't and please let me finish what I have to say. I don't know what you were thinking when you decided to work in Apparatus when you knew perfectly well I was in the middle of working there. That kind of behaviour is not acceptable in our lab, and cannot repeat itself. You endangered me, yourself and the rest of the people in the lab by not following regulations. If you are unsure of how to work with something, ask someone to help you or to supervise you as you work. Now, please take your things and move to the other bench you will be more comfortable there."

He kept nodding and saying "OK, OK, OK." I normally doubt people actually understand when they reply like that, but here's hoping.

He has been avoiding me all day since. Maddie thinks he is afraid to come near our part of the lab.

[artk2002]

Today he insists on working on my bench despite the fact that a bench has been allocated for him to share with another student.
I warned him that I was working on something sensitive, and actually had to put a masking tape line down the bench to mark where he shouldn't cross. I feel like in grade school.
Next time I'm going to tell him that he ruined my experiment and there is no way he is getting near my stuff again, I was just too angry today for it to come out in an effective manner.

You need to understand why is his "insist" was more powerful than yours.

He's doing dominance moves here. If he were a dog he'd be peeing all over your work space. By compromising and not pushing back, hard, you're telling him that he has a right to dominate you. Is that what you want?

I read all of your posts. I do have a problem with being direct because I very often come off abrasive. I'm not good at confrontation.

Abrasive isn't always bad. You don't need Barney to like you, you need him to respect you. And remember, too, that there are times that accusations of "abrasive" or "unfriendly" are just defenses from someone who is in the wrong. They attack the messenger because they don't want to accept the message.

I am the PhD student who has been in the lab longest. Maddie and I share a bench, and we get along well.

What did I say about you having the seniority? Use it. He tries peeing on your desk again, whack him on the nose with a rolled-up newspaper. (Please... it's an analogy, not pet-training advice.)

This morning I told Barney I needed to talk to him, and in a calm, quiet voice told him the following:
"Please take your things and move to other bench. I'm asking you to do this for two reasons: one, the bench is becoming very crowded and two, I am not at ease working next to you.
I am still angry about what happened last week. Your behaviour was unprofessional, irresponsible and frankly, rude..."

Great start!

At this point Barney interjected with "But I apologized for that!"

The defense mechanisms kick in. It's an attempt to turn the issue around on you. He's fine, so you must be the person in the wrong.

I continued, in the same calm, quiet voice:
"Actually, no you didn't and please let me finish what I have to say. I don't know what you were thinking when you decided to work in Apparatus when you knew perfectly well I was in the middle of working there. That kind of behaviour is not acceptable in our lab, and cannot repeat itself. You endangered me, yourself and the rest of the people in the lab by not following regulations. If you are unsure of how to work with something, ask someone to help you or to supervise you as you work. Now, please take your things and move to the other bench you will be more comfortable there."

He kept nodding and saying "OK, OK, OK." I normally doubt people actually understand when they reply like that, but here's hoping.

It's possible that he'll get it, but I think a few more "bad dogs" will be necessary.

He has been avoiding me all day since. Maddie thinks he is afraid to come near our part of the lab.

At this point, fear is good. Someone who works with hazardous materials and procedures needs a little bit of fear in them.

[rashea]

I am the PhD student who has been in the lab longest.  I take this to mean you have no authority over him?

This morning I told Barney I needed to talk to him, and in a calm, quiet voice told him the following:
"Please take your things and move to other bench. This sounds like an order? Is there a rule that he's breaking? Or just your preference? I'm asking you to do this for two reasons: one, the bench is becoming very crowded and two, I am not at ease working next to you.
I am still angry about what happened last week. This is good Your behaviour was unprofessional, irresponsible and frankly, rude..." I think you went a bit far here. You're scolding him. That's something that should be left for his superiors.

At this point Barney interjected with "But I apologized for that!"

I continued, in the same calm, quiet voice:
"Actually, no you didn't and please let me finish what I have to say. I don't know what you were thinking when you decided to work in Apparatus when you knew perfectly well I was in the middle of working there.Good That kind of behaviour is not acceptable in our lab, and cannot repeat itself. Again, this is acceptable if you are in a position of authority in the lab/over him. If not, I think you went too farYou endangered me, yourself and the rest of the people in the lab by not following regulations. If you are unsure of how to work with something, ask someone to help you or to supervise you as you work. Now, please take your things and move to the other bench you will be more comfortable there."

He kept nodding and saying "OK, OK, OK." I normally doubt people actually understand when they reply like that, but here's hoping.

He has been avoiding me all day since. Maddie thinks he is afraid to come near our part of the lab.

[Deetee]

He has been avoiding me all day since. Maddie thinks he is afraid to come near our part of the lab.

Good. You have taught him what should be obvious. There are people that you value the opinion of and care about. This person is not someone that you want to "like" you (a little like elephantchild's neighbour actually) as the only way they will "like" you is if you are a complete doormat.

Respect tinged with fear is what you want.

When I used to teach, for the first few classess I was very strict and a bit "mean". It worked much better because I am naturally superhelpful. But if I started off "mean" the students were grateful for my help instead of taking it for granted.