You can make a few changes to your Guestbook Element settings to avoid receiving spam comments.
Activate the Captcha feature to make sure that your Guestbook doesn’t receive entries created by robots. When Captcha is turned on, commenters have to check a box to confirm they’re not a robot before they can send their comments. Robots fall at this hurdle, since they can’t check the box themselves.
Activate the Moderate feature
In addition to Captcha, you can also activate the Moderate feature. This means that new posts in your Guestbook won’t be instantly visible, and you have to read, moderate and approve new comments first. If it’s an entry that you’d rather not include, you can just delete it, and your visitors will never even see it.