Number one don't return the flirting, but still be friendly. It might also help to mention a boyfriend, casually, while your talking to someone else(even if it's not true) hopefully he will over hear you, and tone down a bit. Just say something about your man taking you to dinner etc. or apply the word boyfriend to something he even mentions. Such as: Oh yeah my boyfriend took me there. If this doesn't work the best thing to do is tell him You don't date people you work with, it's strictly business at the office. If he really becomes persistant, let him know he is making you uncomfortable. Good luck.

Kataroo,

You are asking about an art of conversation whereas you want to cut short (or end) these conversations without hurting his feelings. It's art because there's no one way to achieve this goal. You may just want to apologize and say you're too busy to chat today and keep up that charade until he gets the hint. Depending on the fragility of his ego, it may not take many of those to do the trick. If it doesn't work, you may have to be short with him and risk a little ego damage. But it sounds like the job/career is probably worth it.

One last note, I certainly appreciate your willingness to go out of your way to avoid hurting his feelings. You're a sweet person for having this sort of compassion, in my opinion.

Good luck.