I have found out (atleast this is my experience) that most ISP's dont do anything unless you receive a hack threat, or something serious happens, etc. Thats the only time an ISP has done anything from my complaint.
How about setting up an htaccess on the forum directory and have it redirect them to a page that slams them with 50 popup windows and has a virus .gif embedded in the pages?

It has worked for me in the past, they never came back lol