Suggestions on what to say to a loser ex when he bothers me?

My ex left me around a year ago when I was ill and he screamed mean things at me afterwards without apologizing, even insinuating that I made my illness up just to make him feel bad because he refused to help me. Since then, although he wrote me an email that he breaks contact with me, he has followed and stared at me a few times when we have met out. This has made me hesistant to go to any place I know he goes to. Now, I really want to be able to go to some of those old favorite places with my new friends. What can I say to him when and if he bothers me? I haven't seen him now in six months, so I hope things have calmed down. I do not want to see him or have any contact with him whatsovever, including saying hello. I feel that I cannot act friendly towards him since that will give him the wrong idea.