A bitchy, annoying and clingy friend?

Ok... I have a really annoying bitchy little friend that i'm really worried is splitting up my friend group. At the beginning of the year she started spreading rumors about my friends and telling lies about us (my friends) and her home life. I am in year 8 now, and just found out that in year 7 she was spreading s**t about me behind my back. When she came back to school (after a prolonged unnecessary amount of time off ;S) she seemed to be nice, but soon we realized she was using facebook to tell yet more lies about us behind our backs. She is clingy, and knows that we don't like her anymore, but she still hangs round with us. We can't tell her up front that we don't like her and don't want her to hang round with us/be our friend anymore, because that would result in the teachers getting involved and us getting the blame. Our friendship group has split in two, which I wouldn't mind, exept that my best friend likes and is still friends with the bitchy person. My friend doesn't understand the horrible things the bitchy friend has done to us, and to be honest, doesn't have a problem with it because since she doesn't have a facebook account (and the bitchy friend only abuses us on facebook) she doesn't even get to see it. We want to split off from this friend group, and I wouldn't have a problem with that, but I really don't want to lose my best friend (in that group), especially as she is one of the nicest people you can get. How can we make her understand, and how can we split off quickly and discreetly with minimum feelings hurt?