I just remembered Engadget's uses email validation. You can have the guestbook entries saved to the database, but set to not be shown on the page you. You send an email to the poster, with a confirmation link that will make their entry visible in the guestbook.
If the spammers still keep attacking your guestbook, at least you have their email address, you can block that address (maybe even a whole domain if they have their own), and sign them up at Freepay
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