On almost all of my sites using the guestbook and form module a lot of spam is coming in. Inspite of enabling the captcha function. On this website
http://www.karlrupp.net/en/computer/how_to_fight_guestbook_spam there are possible solutions for this problem. Especially the timestap one looks interesting (but i am no php coder). Maybe one of our php specialists can have a look at this one and maybe give a hint how to implement this in the excisting guestbook versions.