Will she be 18 the next time she sees you?
You are going to have to accept that this relationship is changed.
Either treat her as an adult who happens to be a guest at your house, or you will never have a civil time with her again. This means she comes and goes at will, and sleeps with whoever she wants.
If she needs money, you can attempt to negotiate, but don't ever expect to enfore your moraility on her again.