"I can't do that... I'm... I'm not like other girls."
Of course, the irony is that, in the very act of saying she is different, the girl saying so is being exactly the opposite of different. Perhaps the greater irony still is that, from a guy's perspective, her saying so is actually a pretty good sign that she is about to say yes to sex. Some guys realize this more than others, but the ones that do know that the girls who really are different (in the sense that they don't have sex very quickly) don't feel the need to verbalize that fact in order to convince anyone.
So if you really want to prove to a guy that you are different from other girls, just be different from other girls, and leave it at that.