blocking direct access my guestbook

I am using "Advanced Guestbook" on my site and I'm getting spammed something wicked, even though I am using "human verification". What I have noticed though, is that mostly all of the spam (probably robots) are coming directly to the guestbook from the "outside", and not from my site. Is there a relatively simple way that I can block direct entry to my guestbook so that my visitors can only get to the guestbook from a link on my site?
Just please keep in mind that I don't know much concerning PHP or any other type of programming.
Thank you