dont avoid or regret him try to control your relationship
we all as a human need to love eahother

Lyz hit the nail on the head... Just be cold or sarcastic... be a beeotch and he should get the hint and move on.

pull him to the side have lunch with but him let him know that he makes feel funny he will understand than in the future his approach to you will be a little bit less friendly it has to be face to face private setting no one around to see here anything no hard feelings

If all else fails you can go to your boss (unless he is your boss) and seek guidance. They'll either intervene on your behalf or give you some advice on how to handle the situation. (Maybe this is something the guy is known for and there's a certain way to deal with him).

And I wouldn't worry about your boyfriend. How would he find out? Even if he did know you could make it very clear that you are not interested and have been trying to stop the flirtation.

I'm married and I've been the target of some inappropriate office flirting. I think all you can do it is be cold when he flirts (just don't answer and walk away, don't engage!). He'll think you're a B, but whatever, he'll move on to another girl and all will be forgotten. Just please, don't flirt back. You'll get a bad rep in return.

I would just avoid it if you can. The man who did this to me was offended at my brush off and I had to tell him to stop and things were never the same. Stay out of his line of sight for a little while, he'll find someone else to focus on.

Point out another woman in the office who is good looking and seems like she'd enjoy the attention. Whenever your flirt starts to shower you with attention, just bring up this other woman's name. Put her in the back of his mind and pretty soon he will have moved on to her.