That is truly disgusting. Once you've moved out and got any deposit back, you should report her. She sounds like a right bitch anyway. As to the letter thing, unless you've signed anything saying that you agree to abide by a rule that says you can't have visitors, I can't imagine you have any legal obligation, so as you're not bothered about good will, I'd say have your friends over anyway. And if this girl's parents try to say anything, or withold your deposit, you can sue their asses