Since this girl is christian, and you cant afford to move out... right? Anyway... force her to move out. Start performing 'realistic' satanic rituals in your rooms. As long as you dont really sacrifice anyone I can't see any trouble coming from that besides her blabbing to everyone she knows about how you do satanic rites. But she obviously blabs all the time anyway. Let her find you with a chicken in the middle of a circle or star. Candles lit and just talk a bunch of mumbo jumbo. Look at her with an evil smile when she catches you. You could really have fun with her and unless you're going to a strictly catholic school, I can't see any real harm coming of that. Religion is your choice and if you choose to call upon 'Satan' as your 'god' that is your choice.
Sometimes if you mess with people's minds enough, they will eventually give up trying to involve themselves in your buisness. Especially if it seems that there is 'no- hope' for your salvation.
Instead of allowing her to get you so angry and upset, get back.
Remember revenge is a dish best served cold... and if the satanic rites don't work... there is always the old Nair in the conditioner bottle trick... but you didn't hear that from me.