It might be time to revisit this idea.
Actually Skkorp has tried this before and does not like it, nor do I. The reason being is that it makes the board less attractive to new legit members. I've been on the other side, and it is annoying to know you want to register on a board and post a reply and not know when/if you will be approved.
The problem is not the spambots. The image verification and email verification improvements have pretty much reduced it to almost nil. The problem is spam houses from overseas countries that use low-wage workers to manually spam, which any robot defense can't prevent.
We've been hit from Nigeria, New Dehli, Samoa, Russia, etc... So what we are doing is one-by-one blocking the spammers from a known subdomain that are not used by a regular user. So far, it seems to be working and the spam is slowing down to a trickle.
If you see a spam post, you can help the mods by reporting the post. Just click on the
icon under the user name, and a moderator will be notified to their email regardless if they are on the board at the time or not.
Thanks!