First off, lifesaver, you're not out of line. It's disrespectful to everyone who worked so hard to earn the right to wear/display your letters, that someone should just slap a sticker on her car etc. My sorority doesn't even let new members wear letters, because they don't know what they mean yet - you have to be a fully initiated sister.
Now I gotta share my roommate horror story...
At my school, dorm space is at a premium but everyone is guaranteed 4 years of housing. So freshmen are usually put into crowded rooms (3 people in a double, 4 people in a triple etc.) I met 2 really nice girls during rush, and we decided to room together. We wound up crowded into a "cozy" double.
Roommate #1 had the same first name as I do. If someone called for me and she answered the phone, the caller would have to ask for me by last name, or she would assume the call was for her. She once talked to one of my sorority sisters for 5 minutes before realizing this woman was my sister and not hers.
(We were all 3 in different sororities)
RM #1 also came in as a freshman but with a lot of college credit. The school wouldn't give her credit for some of it. So she spent the entire semester whining about how she shouldn't have to take freshman classes with the rest of us.
RM #1 also had to study to music, however the concept of headphones was totally beyond her.
Roommate #2 was a total neat freak. Our desks were next to each other, and if I had a piece of paper on my desk where one corner overhung by 1/4 inch, she would scream at me.
The trouble really began when RM #2 began dating a guy who lived down the hall. Not only would they be having sex in the bottom bunk with me trying to sleep on the top bunk... He was a total racist, and he turned her into a racist too. He, she, and RM #1 were all the same race; I'm a different race. So before long, RM #2 wouldn't give me the time of day, but would spend hours chatting with RM #1, usually while I was trying to sleep or study.
I complained, and, long story short, got a single in the middle of my freshman year. Which did not endear me to the rest of my floor, since it was a big room that usually went to a junior.
Anyway, eventually my ex-roommates moved off my floor.
After that, the worst that happened was when my suitemates continually used my pots and pans without washing them afterwards. I eventually had to start keeping them locked in my room...
...and now I have the best roommate of all... my husband