At that point you can tell her that she ruins your clothes and that you don’t/didn’t appreciate it. So she lost those "privileges", if she gets defensive about it, you can always give her one more chance but if she ruins it, she pays for the clothing. Or completely tell her no and if she’s defensive about that honestly consider a new friend.
(2) I’m a generous and kind friend and usually don’t have a problem with people borrowing stuff. But it’s getting to the point where I’m anxious for the school year to start because I won’t have a good excuse or reason to tell her no she can’t wear them. But I know I absolutely don’t want her wearing them. Also they are not cheap clothing they are expensive which is another reason I don’t want them ruined. How do I tell her no without being a bitch?