I think you've just discovered what most people who have guest books on their sites discover...
1. That there are some very strange people out there.
2. That guestbooks aren't really a great idea.
I'm afraid I wouldn't have a guestbook anywhere near any of my sites. I've never really seen the point of them, any more than I see the point of having them in hotels/guesthouses.
It's not difficult to visit a site without leaving an IP address, by the way.