It is better than captcha.

Get fly06 spammer blocker...easy to set up and a life saver for my site extensions.joomla.org/extensions/access-a-security/site-access/3924

Registration Validator (the free version, or the paid version) are also perfect. My old domain was hit hard by spammers for months, preventing me doing anything much as they were using Xrumer bots.
www.cedit.biz