Crazy girl trying to steal my boyfriend>:(! What do I do:(?

Okay, so she arrived to my class in the first of the year and I thought she was nice, u know, typical good girl;), but on the first day I found out she had liken my ex bf, lets call him David, and he said she was an ugly girl and all that s***:/. Then she just changed and started to get into trouble a lot and so I avoided her a lot, and my friends said that it was good I left her alone because she was a b**** and stuff:). Then she started to sneak around with all of the boys being a w****. And then, me and my ex bf got back together and we are currently dating.... And recently I had found out she has been saying stuff about me and David dating>:((! My bff (lets call her Mary) then told me crap about her saying stuff about me like "Ew why does David like her she is a b**** and she is evil and ugly as h***." Then I got really p***** after she told me more like she said "Omg David is so cute Imma ask him out hehhe." I really got mad and then confronted her like "Uh excuse me are u thinking of asking out David???". And then she went all. "Shut up about it before I rip your hair out!! And yeah I am!!" and all I did was ask nicely!! Then I really got p***** and said "Well maybe you should stay away from whats mine!!!" and left her alone. And lately she's been saying stuff to Mary like "You need to shut up because David belongs to me and not Stella (lets just say thats me)" and "Oh come on Stella's such and ugly w**** b**** and she needs to go to h***." She just told me this yesterday so I'm obviously still VERY steamed about it!! And now all she does to me is roll her eyes and whenever I ask what are you mad about she'll be like "You and your freak boyfriend!!!!". What do I do to get her to stay away from my boyfriend?!! Thanks for ur advice :) Xx