I rejected this guy who was my friend. Now he's a jerk. What should I do?

Defiantly delete him of Facebook and im sure if you block him on Twitter his messages wont still come up. He clearly still has feelings for you or he wouldn't bother talking to you. The fact is people can become very obsessive over someone so if it really starts get seriously (him stalking you, abusing you badly) then i would tell someone about it. I say confront him and just ask him to leave you alone even though your moving, the abuse will still carry on on the internet.