Those bots are mercifully stupid. I think most spammers are happy hitting the 90% or so of unprotected sites, and as soon as you go for a custom built registration your no longer targetted. I hope.
I have found that using an unconventional registration system helps. Also, using forum software that not many people use keeps the spam down, as most spammers only target the most popular software (like phpBB). Its like using OS X or Linux to avoid all the viruses targeted at Windows. I particularly like punBB, which has excellent anti-spam mods.
For a Navalism simulation game i also run a forum, and it is based on SMF (simple machines forum), that is also very low on the targetting list for spammers apparently.
I guess the modified registration procedure also worked for you, ive not seen any suspicious users apart from robotslive.