Roommate Problems That I Need Help Solving - Comments

Thanks, guys (: You both gave really good advice that I've actually been considering for the past few days, I just wasn't sure if it was the right thing to do, or if I was overreacting. Thanks again!

You should approach your resident assistant about it and see what they have to say. She can help you mediate.

I'm not sure if you can approach this without her feeling insulted because
really, she's the one being rude and disregarding the fact that it's not. HER. Futon.
Either way, she may feel as if you're being "unreasonable" but don't back down, ok?

You explicitly state it is your futon yet you have allowed her to use it (and ABUSE IT).
That in and of itself already made your roommate believe you gave her partial
ownership which isn't the case at all since you want her to cut the crap.

Just tell her in the most polite way that you can that " would appreciate it if you
would not take advantage of the fact that I allow you to use my futon. If you would
like to utilize it, please keep it sanitary and available for my use as well. I don't
mind sharing it but be respectful of my belongings as I am towards yours."

Good luck with it. I know you will figure this out.