Don't know what you're programming level is, but adding a psuedo password phrase as a requirement to post a guestbook entry helps. The bots can be good, but they aren't that smart. I have a site for someone to post baby pictures (1200 at last count

and there's a guestbook feature i wrote there. It's no fancy thing, just leave your name and a comment. Spammers started hitting that pretty quick. I added an entry to the guestbook form asking to 'what is his first name'. No where on the page the form is on does it list his name. Only on the header of a previous page. Haven't had a spammer since. And that's 8 months now or so. (knock on wood)

lol