Guestbook abuse

Hello
I have people entering my guestbook with advertsiments of their own web sites. Usually porn and other none suitable web sites. I would have thought this is illegal. The IP address does not show, so I do not know how to stop it. Can you please help me?
Thank you in advance.
Andrea

If you read the sticky thread at the top of this forum, the one titled read this before posting advanced guestbook questions you will find links to pages intructing you on how to add a simple anti spam system to the guestbook and other modifications.