As I say, I've already said variations of this about five times - including the requirement to pay me if they want/need my services. I'm not a person to shy away from telling someone something (after all, I did just dump them for late payments). But even after telling them, I get another question (completely ignoring my past comments regarding payment).
Unfortunately, it's near the point where I'm going to have to turn up the heat, or just start ignoring them. I don't want to have to do this though, as it just makes me look like the asshole, and perception is everything.