How to deal with this "mean girl" and mutual friends?

So there's this girl. We were friends, but several times throughout our friendship she has been rude and nasty to me. Saying terribly offensive things. So recently, she said some extremely hurtful and cruel things. Later we made up, but I specifically said we would just be school friends. So now school has ended, and I unfollowed her on Twitter. She got very mad and I saw her tweets saying that it was uncool of me to unfollow her. She called me a snotty little rat, a b!&$h, etc. I blocked her. I texted her to "clear the air", saying I told her we would just be school friends. Then on Twitter, she said she wished she could slap me, she called me a b!$&h again, she said she didn't know how I had friends, that I was stupid, and she hated me. She really is cruel and toxic. But this girl who is supposed to be my BFF doesn't care and wants to be her friend even though this girl is hateful and is just using her to hurt me. Advice? No mean comments!