How do I get this boy to leave me alone?

There is this boy at my school that I used to be friends with. Recently, him and my best friend started hating each other various disagreements. I'm not sure what happened and I don't try to get in the middle, but I do know that this boy enjoys annoying people and he doesn't have the best set of social skills to be found. There relationship is very bad; she leaves nasty (full of profanity) messages on his phone, and he does the same to her. Recently, in social studies, we were igned groups, and both this boy and this girl were in my group. I tried asking the boy to leave the girl alone, and he agreed, yet he ceased to stop talking about how much he hated her, and he was getting in the way of our work. The two of them were constantly at it. Finally, another group member and I told the teacher there needed to be a switch, and the boy got switched out. Now, he emails me all the time telling me what a jerk I am and how I had no right to deal with "his business." I try to telling him to stop but he will not stop. What do I do?