Replying to Ex boyfriends.. AHHH

okay so i dated this guy for like, a week and then i dumped him because he got annoying and clingy and stuff. i talked to him about it and he wouldnt stop so i broke up with him. but now he's like EVERYWHERE i am. it's like he's stalking me. and its really hard for me to do anything with him like, watching me. now he's trying to be like best friends with all these people who i hang out with and it's really annoying. what should i do because talking to him about it does not help whatsoever...?