For my part, I'd go with Aikido... If you're not that strong , it's one of the best martial art you can learn, imho.

But anyway, you shouldn't have to. I really doubt a guy will come all the way from where-ever he is, to simply try to kick your ass. But as pooh sun tzu said, if he keeps bothering you, go to the police. Don't let a low-life scum bag keep you up at night.
Alternatively, you may want to change isp... although the probability of him getting your real address is, I would say, pretty low.