The problem is that the forums are using an out-dated version of the forum code. A simple upgrade to the most recent version, including a more secure CAPTCHA, and installation of an add-on or two will stop the nearly all the spam automatically with no need for human input. I was working with another forum (run with phpbb) that was getting spammed like crazy, and all I did was install an add-on that made it so you need to have made at least 10 post before using the url tag, and I haven't seen any spam for several months now (lol knock on wood). For phpbb the upgrade process took probably less than 20 minutes, and I don't think SMF (the code this forum is using) will be much more difficult.
These spammers are coming from automated bots, likely NOT real people, that are exploiting vulnerable versions of the forums..