I guess you may have to tell us the certain reason why you have to interact. Do you work together? Do you have kids? I can’t think of any other reason why you would have to interact.
@GabrielsLamb has hinted that you and this guy have an icky history and that you may have gotten back together with him at least once. I have no idea whether that’s the case or not. If you have in fact gotten back together with him, even if it’s just one time, then he probably assumes that you might be willing to get back together with him, even if it’s just for a booty call.
If you really don’t want to be with him (and others on this thread, who may have more knowledge about the situation have hinted that you still like him) then you need to figure out if you really do want to get back together with him.
I’m going to go out on a limb and guess that this guy is a real creep and that he either dumped you in an ugly and dramatic fashion, or that you dumped him because he did something awful (like cheating on you). If you guys broke up because he’s not a quality guy, then you have to come right out and tell him that you don’t want to talk to him or see him or have any other contact with him (unless you have to because of work or having a child together) In that case, simply be polite, but distant and don’t play into any of the potential crap. Don’t get angry at him, don’t engage him and don’t fall under his charms if that has been your problem in the past. Douches don’t turn into Princes. Just doesn’t happen.
But like Auggie said, if he’s over the whole ugly thing, and you have to interact occasionally for whatever your certain reason is, then he might just be trying to be polite. Even if he is up to no good, just don’t buy into it, don’t engage him and don’t think about “giving it another try.”
Is it possible that you could give us a few more details? Otherwise we’re all just speculating here.