In the most recent versions of 6.x, 9.x, 10.x and in 11.0 there is a new feature. Just visit tiki-admin.php?page=security -> Spam protection and activate "Require passcode to register" and "Show passcode on registration form".
CAPTCHA seems to do nothing to slow them down and annoys humans.+1 to integrating specialized solutions like Bad Behavior, StopForumSpam, ZB BLOCK or PHPIDS
@nikosal25: "question and answer field, like I had done in my mediawiki site" -> This is indeed very efficient because it's contextual to the site.
We just need someone to take the lead on this. Here is how to get commit access.