Sure...

Subject: Sure...
Name: SickOfItAll
Date: 6/6/2001 9:23:07 PM (GMT-7)
IP Address: 24.16.212.62
In Reply to: Is there anything I can do? posted by Ben
Message:

Get a nice log of the activity, and contact his ISP. Most ISP's take this activity very seriously, and will make sure it stops or cancel his service (without revealing your identity).

If the IP keeps changing, its very possible that he's on a dialup ISP, which assigns a new IP every time he connects. That or he's going through a pool of proxies somewhere. Do a tracert to see if they all come from the same general network.