You can create and embed a guestbook for your website using the built-in Comments App, in the UltimateWB Full version software!
Check out the UltimateWB Video Tutorual, "How to Add & Customize a Guestbook on your Website", for a quick, easy way tro add a Guestbook page to your website using the built-in Templates app, and how to customize via your website admin panel.
And you don't have to worry about spam posts ruining your Guestbook section on your website - you can set post approval settings in the Comments App, besides using the CAPTCHA form feature to help prevent spam.
Follow the steps outlined in the Comments App tutorials:
Add Comments & Ratings Section to your Webpage
Configuring Comments & Ratings Section: Comment Like/Unlike, Subscribe, Post Processing