I wonder if it would be wrong/too awkward/to blunt to go at it like this:
Guy: Wanna go out for a movie?
You: Would this be a date?
To which either he replies yes and then you tell him something along the lines of "Sorry, you're a very nice person, but I'm not looking for a relationship
right now/I'm not attracted to you in *that way*", or he replies no and then you just smile and say "It's good we've set this-one straight".
A more subtle way would be:
Guy: Wannna go out for a movie?
You: Great idea. We could call up some other folks from our workplace/common group/any common friends and go all together.
By this you're signaling you deffinitely don't think of going out with him as a date. If he says something like "I'd much rather go with you alone" then he clearly means it as a date and you can tell him "Sorry, you're a very nice person, but I'm not looking for a relationship
right now/I'm not attracted to you in *that way*"
If you are in a relationship
/engaged/married, mentioning in a very appreciative way your boyfriend/fiance/husband every other sentence works pretty well too... (Worked for me a few times, when I was in a relationship