What to say to avoid further problems?

I am ending a friendship with a gossipy backstabber who thinks I'm a super hot topic. I seem to be all she can talk about to anyone and its never good. She is supposed to be my best friend and she still calls to set up girls nights and yap about how awesome she thinks she is. What do I say to her when she calls? I need goos excuses so she doesn't erupt into a gossip whirlwind slandering me all over for dropping her as a friend. I already lost my job because of her crap I don't need anymore problems