Ya... contact your ISP, tell them you website is getting spammed... contact the Guestbook provider, tell them the guestbook is getting spammed...

Delete the spam as it comes in, but COPY EVERYTHING to a text file or something so you have a record, or just leave it there until The Powers That Be can get a look at it...

and yes, do NOT engage in a flame war with the spammer... If you ignore idiots like this long enough, they get bored and go away...