How do I deal with my brother's stupid girlfriend?

She drives me NUTS. She just goes into my room and borrows things and doesn't even ask. For example, right this very second I just saw her out the window go out and she had MY motorbike helmet on to go on her friends motorbike. WTF?! Who does that?! My brother doesn't say anything - he just thinks its fine. She doesn't even ASK. I've taken it up with my brother and other members of my family yet she continues. She's generally a very nice and sweet person but sensitive so I don't want to approach her. I get VERY angry with people VERY easily so I'll end up screaming at her and punching her in the face or something awful like that. I've tried not leaving my stuff around and putting it upstairs locked away in my room but she still manages to get stuff. Whatta tramp. I get so mad at her. What can I do to relax and stop her from taking my stuff?