The user already registered successfully, so I was offered a solution to ensure him/her do not go through captcha trouble again when logging in on their account. I do not know of any measures for users who are trying to get registered on the forum.
An immediate solution that can help cut down on the number of those bots is there but I don't think Theymos can implement it. Not anytime soon.
making email verification possible as one is registering. Now I know people will say that creating emails is very easy but this can give the bot accounts creators some painful experience and others will give up eventually.
This could go along with periodical suspending of accounts engaged in some sort of suspicious activities or bot spamming. Bots might not have time to check their emails and the numbers will be further slashed down.