this guys being awful since I stopped being friends with him ? help ?

[Samantha]

I used to be friends with this guy who always bitched about everyone and basically acted like a very catty girl

lately I've realised how much his negativity was bringing me down so I decided to try and get him out of my life in the nicest possible way

I stopped starting all conversations with him, avoided him when I could and only spoke to him when he spoke to me. After about a week he took the hint

in one of the classes I am in with him he sits next to this girl and the whole lesson they sit and ***** about me loud enough so I can hear it, it's usually very hurtful comments for example I've heard him call me anorexic which has always really upset me

he also posts bitchy things about me on twitter and our mutual friends have told me he stalks through my tweets even though I have blocked him he has also told them he is going to he "revenge on that *****" ( me) and the girl he sits next to in computer now wants to beat me up please help I don't know what to do