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What version of XF Guestbook are you using?
The latest is version 2.40a and can be downloaded
here.
Note that the verification code doesn't work if you're using Protector.
For me the best option was to change the name of the sign.php to something else, just to fool those spam engines. You also have to do this in the files
include/form_sign.inc.php and
templates/xf_guestbook.html.
I've requested theCat a while ago to have an option in the admin panel where you can enter the filename of the signform, so you don't have to go through all the files and update everything. Changing the filename of the signform on a regular base is a good way to stop the spam.