If you are NOT using the GBook script the solution is obvious - download and install the GBook guestbook script from PHP Junkyard.

If you ARE using GBook please check the following:

1. First and foremost: make sure you run the latest version of GBook as SPAM protection increases with each new version.
    
2. Enable Captcha (anti-SPAM image) - make sure $settings['autosubmit'] is set to either 1 or 2 in the settings.
    
3. Write a good anti-SPAM question (setting variable $settings['spam_question']), see readme.htm file for details and examples.
    
4. Make sure you have JunkMark™ filter enabled - $settings['junkmark_use'] should be set to 1.
    
5. You can choose to manually approve new guestbook entries by setting $settings['man_approval'] to 1 in the settings.
    

You can learn more about what SPAM is and how GBook fights it in my Guestbook SPAM article.