KINETIC PRESENTATION SYSTEMS

At the Melbourne Joomla User Group in May 2010 there was a discussion about ways to prevent spam on Kunena forum component.

Kunena captcha setting is a setting that only effects the system when public posting is allowed (which in most forums is not recommended). So to fix it, Captcha needs to be implemented at the user registration level. Kunena uses the main Joomla login to register users, so you need to change the registration page for Joomla to fix it.

Kunena posts:
http://www.kunena.com/forum/4-archive-support/2921-captcha-on-but-where-is-it
http://www.kunena.com/forum/133-solved-or-prior--common-questions/32752-clear-instructions-on-using-a-registration-a-login-captcha-with-kunena

So the best way to then add Captcha to Joomla registration is via a template override - where you create a copy of the registration page in your template folder, and then modify it's layout there.

Fortunately, someone's written it all up...
http://community.joomla.org/mag-v1/article/550-add-acceptance-terms-to-the-joomla-15-registration.html

You can also look at using recaptcha as a captcha tool, and also look at other forms systems like RSForms, Chrono Forms, Fabrik and other extensions for user registration to also meet the needs to prevent spammers signing up to your site.