The best way to deal with the problem would be to beef up the registration process on your forum.
A simple way to do so on 1.2.* is here:
http://punbb.informer.com/forums/topic/ … e-mod-v08/
Another simple way to do an anti-spam registration page on 1.2* is to download this file by Jacky:
It includes a ready-made anti-spam register.php you can just drop into your site to replace the default register.php (and a few support files you'll need to place on your site too).
You will need to customize the questions the mod asks though, of course, but that really is pretty easy and doesn't involve editing any PHP code.
You can also search for 'spam' 'captcha' 'spambot' on this forum and you will find a LOT more discussion on the subject of PunBB and forum spam, and a galaxy of ways of alternative ways of dealing with the issue.