It already has email verification now, so I don't think a captcha would offer much more help. I'm all for trying it, but I wouldn't expect it to be any good.
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.
They usually link to an IP, not a domain, they have a very, very large botnet pool of IPs, and even if they use a domain, not all countries respect our views on spam and the domains can be registered there.
It's not often that simple either. Some of the ways these spammers profit is to sell traffic to a domain to boost the domain in google rankings to increase its resale value (who do you report it to? google doesn't like to listen and the domain will be sold to the victim by the time anything happens), or selling scam goods like illegal software on a site they set up in 10 minutes and don't care if it's taken down. Or they force malware onto your computer and get paid a per-download fee from the malware authors.
Fighting spam just doesn't work on the level you're talking about, by the time you track them down and get someone to do something about it, they've moved on 10 times already.
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 