If you really want to get rid of her for good, the following is effective: “You were the worst lay I’ve ever had and I can’t believe I ever slept with you.”
If a girl tossed that to me, I could answer 2 ways:
A) Yeah, right. So how come you never said no?
B) Too bad for you. I liked it.
Pull a big shitty grin after that. =D