You can disable writing to your guestbook from those IP addresses that are causing trouble.

The easiest way to stop unwanted messages from a specific IP address is to go to the Guestbook Messages (m) section of the Guestbook Management page. In this page you'll see the messages that are in your guestbook and the IP addresses from which each message was submitted. By clicking the 'Disable IP' button the specific address is blacklisted ( i ), and after that it is no longer possible to send messages to your guestbook from that address.

Addresses that are blacklisted are colored red in the 'Messages' page. They can be enabled again by clicking the 'Enable IP' button.

If the problem is that the same message is flooded many times filling the guestbook, you can increase the time limit value (in the Abuse Protection (m) page). Time limit specifies the time that must pass before one can send another message from the same IP address again. There is no minimum time limit, but is advisable to set the value to at least one minute.