They're exploiting a known YaBB vulnerability, the attacks will stop with captcha added into the registration process. The board isn't just being spammed, it's being flooded - captcha will plug the hole enough for it to stop.
Interestingly, my site was 100% custom code. I had what was effectively a multistep process to post anything, the data entry form generated a unique ID that had to be submitted back with the form and each ID was good only once. I thought that would stop automated spamming, but it only slowed it down very briefly. Even when I changed the format of the ID to try and break a parser it made no difference, so I really don't know what they were doing. When I completely deleted the php file that had the code to write my database, the spam stopped instantly. When I put it back a few weeks later, it restarted almost as instantly.
Unfortunately the spam has disappeared, next time it shows up (if it does) I'll take a look at what they're "desperately" trying to tell the world about. They're not impervious to detection, that's why most of them get caught... eventually
Wow, how'd you get it to disappear?
I have a moderated area where posts don't go live until I approve them, I've been ignoring it because I have better things to do than delete spam, but it's growing by about 300 spam aday right now. These posts never show up anywhere on the web, not even a hidden admin page, yet they keep flooding it.