This isn't a situation or a question (so please, no "what's the etiquette question here?" posts), but some thoughts that I've had for a while that I think bear putting into a summary post and discussing.
We frequently (as in just about 2-3 every day) get posts that can be boiled down to this:
"I have an unreasonable person in my life who does something that I don't like. If I assert myself they'll get upset and up the ante making me and the people around us uncomfortable. What polite words can I say that will get them to change their behavior?"
What the poster means by "polite" is really "won't set the other person off." The sad truth is that those magic words don't exist. If the person were reasonable, then there wouldn't be an issue. Polite words would work fine. With unreasonable people, we're not so lucky.
The only choices are to remain silent and put up with the bad behavior to keep the peace, or assert oneself and put up with the fall out. Although we encourage people to stand up for themselves, neither choice is wrong in-and-of itself. They're only right or wrong for the specific context. I've chosen both ways in different situations. Asking yourself "Is this the hill I want to die on?" is part of the process of making that choice. That said, I have a strong preference for asserting oneself, for a couple of reasons: First off, if things are going to be uncomfortable whichever choice you make, why not be true to yourself? There's great satisfaction in knowing that you stood up for yourself, even if it brings some discomfort. Second, there's a chance for change. If you say nothing, change will never happen. If you do say something, it's possible that the bad behavior will stop, eventually. Search for threads where "polite spine" has been effective -- there are lots of them.
If you choose to assert yourself, there are a few "dos" and "don't" that should be observed. Don't JADE -- Justify, Argue, Defend or Explain. Explanations and justifications are great with reasonable people. With unreasonable people, they're just an opportunity to negotiate. You get into exchanges like this: "I can't drive you cross country tomorrow because I'm having a hangnail removed." "You can reschedule your appointment." "No, I can't, it's taken 8 months for the health service to get this scheduled." "Aw, don't you care about meeeeeeeeeeee? We can leave right after your hangnail removal -- you'll just have to drive faster!" That kind of conversation is endless -- I've heard it likened to trying to squeeze a water balloon. Every time you push on one side, it bulges out on another.
Do keep calm and try to keep emotion out of it, even if you're hurting. Accusatory words will only put the other person on the defensive. The "I message" is a very good technique: "I <feeling> when you <unwanted action>; please <desired action>". That won't necessarily get the other person to change, but it has the best chance. Saying "no" or "stop" or whatever isn't rude in and of itself. It can be done rudely or politely. "Please stop <x>" is polite. "Lay off, you sheep-bothering, mouth-breathing moron" is not, no matter how satisfactory it might feel.
One thing to remember: Just because someone else gets upset, it doesn't mean that you've done anything wrong. It's always a good idea to do some self-assessment, but don't end up taking responsibility for other people's feelings. If you do something and it sets someone off, ask yourself "If I did that same thing with other people, would they go off, too?" If the answer is "yes," then you need to change your behavior. If the answer is "no," then the issue is the other person's, not yours. When in doubt, come here and ask; we'll set you straight! Please don't take this as if I were saying that you should ignore the feelings of others, but you should be careful about whose opinions and feelings you care about; it's very context-dependent. You should care what your boss thinks about your job performance, but don't need to care what s/he thinks about how you relate to your spouse. Your mother-in-law can care about how you treat her and (to an extent) your spouse; she doesn't get to care about how you do your job. The stranger at the bus stop? They don't get a vote at all. I know that it hurts when a stranger comments on weight, or parenting or appearance or anything, but you have to ask yourself "why do I care what that person feels?" Make sure that when you're looking for the magic words, you're looking for the right reasons. Don't waste your time trying to placate someone whose opinion, in the long run, shouldn't matter at all.