if the guy is a creep, I generally blow him off/ignore him. if I like a guy, I will not just tell him I like him unless he has openly said he likes me/has asked me out on a date. I suck at telling guys I don't like them, especially if I like them as a friend. I usually don't make an effort to talk to them as much like fb or exchanging numbers, unless he has to get my number for class or something... see, but what's messed up about that is that I get really shy with a guy I like, so I may do the same thing - give him signs that I don't like him and signs that say that I do like him with signs that say we're friends lol
mixed signals can sometimes be unintentional ways of defending yourself against rejection. if I stay at the cusp of friendship/romance, I can bounce back to friendship easily if the person isn't interested and I can also pursue the romance if they are interested. I try and get that balance of flirtiness/friendship with a guy I like and I stay friends with the guy I don't like.
what is this girl doing that makes you think she doesn't/does like you?