Angora Guestbook uses several techniques to fight spam :
* CAPTCHA, to prevent robots to sign the guestbook
* Anti-flood, to prevent humans or robots to flood the guestbook with lots of void messages
* Random fields name : this will generate a unique form on each sign page opening, thus limiting robots from flooding
* Post moderations, so you can valide a post before being shown
* Auto-censor messages : posts containing any censored word will be automaticly unpublished
* Ban IPs : you can ban individual IPs or a range of IP, to prevent spammers from flooding the guestbook
As always, all these features can be configured or turned on/off in the admin center.