Maybe you could reverse scan his machine, start finding out some information on this guy; See if he has any DNS entries, and if so a contact email somewhere.

What you should do is just straight up complain to his ISP and get him cut off.. chances are he's a script kiddy running a windoze scanner. Scan him back... If he fingerprints a windoze 9x box, call the po - po (actually, his ISP I mean.. hehe)

If he's at least.. probably using nmap and linux, and you want to be 'nice' then do a dnslookup on him, find some contact info somehow, preferably an email, and start there.. hopefully he still uses the e-addy from his entries. If he doesn't reply, or even replies with "I dunno what your talking about" crap, well... I bet you he'll stop scanning your machine.