This is an ancient thread, but it's a common theme and I think the advice to this question hasn't changed. If a girl tells you that she has a boyfriend, you can't ignore it. It could be a sign of low interest or she could just be letting you know the deal, in case something does happen between you.
I would rather that a girl not tell me if she has a boyfriend. I don't ask and I don't want to know. But if she does, then you have to acknowledge it and let her know what your position is. For me - I don't give a sheet really, as long as she is willing to bang. If not then I don't want to deal with her. So I tell her "if something changes and you want to do something together, call me". That makes it clear that I don't want calls from her unless she wants to get together, but on the other hand I'm not just blowing her off.