Why would he do this?

soo this all started happening again when my ex added me on facebook i accepted my fault just trying to keep the peace and when he made a joke one of my other guy friends go into a fight after the fight he was saying he was sorry for the problem and my friend kept making it worse i just tried to stay in the middle and tell them both to shut up after my ex UNFRIEND me from facebook i was kinda mad but didnt care as much after that he seems to go back to having my name always in his mouth he will just not shut up about me he tries to make people dislike me especially on facebook and the people that dont think its wrong i have stopped being friends with when we had our last day of school he always seemed to try to be around me and i stapped a couple of times on him cuz he knows i hate when he is around me and yes i know i could of handles that better I DO NOT WANT HIM IN MY LIFE!!! ESPECIALLY SOPHMORE YEAR!! but i do no know how to get him to keep my name out of his mouth and just leave me alone since then i havent had any contact with him why wont he just leave me alone?