Just ignore him, don't talk to him, don't hit him, don't do anything. If you have 9 or 10 villagers in your town he should be the next one to leave. If you have less than 9 villagers, you need to wait for another one to move in before he can decide to go. He'll either tell you, or what you can do is talk to one of the villagers you like for 10 or 15 times, he should tell you who is the next one who is planning to move, so you can make sure it'll be him. Anyway I'm not good at explaining, you can check this topic, you'll have all the details :
http://www.belltreeforums.com/showthread.php?82107-Guide-Fool-Proof-Guide-to-Moving-Villagers-OUT