The first step to preventing spam posts of this nature is to enable CAPTCHA on the guestbook:
  1. Go to Set Up > Site Options.
  2. Search for BLOG_CAPTCHA_ENABLED
  3. Enter a value of TRUE in the field and save your changes.

While we make every effort to control spam, some spam is sophisticated and can still pass through CAPTCHA checks. In this case, if you are still receiving spam after enabling CAPTCHA, you may consider deleting the spam posts or disabling the guestbook all together.