Best friend problems .........?

My best friend is acting like nothing short of a tramp and alot of people dont like her in my school and all the people that dont like her dont like her friends and want to fight her and her friends me personally i dont know why i am being her friend right now but i cant help it. The people that wanna fight her give me crap all the time and i dont know what to do about them. And my friend is being really rude to everyone and im sick of it but when i try to stop being her friend by ignoring her and telling her to back off she continues to talk to me and call my house and my cell and i try different techniques to get her to leave me alone none of them work what should i do?????