I can't get away from her - help! ?

There's a nutcase girl I used to work with, we were friends before but for some reason she started changing her attitude towards me. I can't think of anything that would make her this way with me. She acts like a total B-word all the time and always tries to find ways to irritate me or bring me down. I would usually clock on to what she was doing and pretend to ignor it. Why should I react and drop to her level? Anyway I left that job (not cos of her...wasn't happy in the job anyway!) And she's still creeping in to my life somehow! Even bigger problem is that she's with my boyfriend's brother. This girl is poison and a gold digger from the way she treats people. Cos of this situation I've already tried being civil and that's not working still. Before that I already tried being friendly. But this is her problem against me!!! I've done nothing. Jelousy perhaps? Either way this girl is always around, she's changed my partners brother in to a D*** and she's just got this thing against me! I feel this way because of the way she treated me before. Can't say on here because there's too much but I don't think being friends is possible. I tried, she's a child and will not listen to anybody. She drives me crazy and I don't know how to deal with it. What would you do?