Help in preventing spammers from leaving inappropiate matier

I am maintianing a web site for a home school support organization and am having trouble with someone spamming advertisments for in appropiate web sites into the guestbook. I am using the advance guestbook 2.3.1. I have tried the ban IP feature, but the adress is not a valid IP and the ban IP wont take it.

I am at a stage where I have to remove the url option or remove the guestbook from the site. Any help would be greatly appreciated.

Joe