Unless the guy has a static IP , your chances of finding them are slim at best.
You can narrow down what ISP he is using (as long as they aren't spoofing an addy) and complain to their abuse dept.
I have been getting a LOT of FTP hits on ZA from the France/Germany area, and sent a complaint to the ISP that hosts mosts of the addys, and got a reply back to the tune of "we don't pursue complaints of port/ping sweeps coming from our networks"
Thanks a lot..
You might have better luck if they are using a U.S based ISP.