How to play a player (who's also my best friend and flatmate!)?

basically I have been sleeping with my flatmate and best friend for the past couple of months...though she has a gf..i know, i know! As it was becoming more hle than fun I said we should put a stop to it, she now says she wants it all but doesn't know how to get rid of the gf at the moment. I have now been playing it cool, not responding to her flirtations etc. Is it best to reject her if she tries it on so as not to let her hav her cake and eat it or will this backfire? Don't want her to think I'm totally uninterested incase she gets her act together but also don't want to make it easy, any thoughts please?