while doing a whois on someones IP, it will show you their system admin and their phone number. Give the system admin a call.
I wouldn`t recommend calling. Email is the way to go. ounce you have found out the ISP of the "attacker" send a detailed email to the [[email protected]] address of their ISP. Make sure you include date and time of attack, your logs, and anything else helpful. If you don`t they may/will file your complaint under J. (junk). After doing this,and have sent the mail some ISP`s will send you an automated mail explaining how they handle abuse problems.

...Just thought i would throw that in.

Mike