SABRE has a lot of Image confirmation/Captcha option and a E-mail confirmation option to prevent spambots from registering for your Blog while keeping real users from being able to join your blog very easily.
This plugin will prevent your WordPress Blog from being taken over by automated Spambot registration, so your MySQL database won’t be flooded with fake users and spam comments from spambots if you have your blog set up to only allow registered users to comment on your posts. This will prevent you from wasting up MySQL database space which you are paying for, and you would not have to spend as much time deleting spambots and spam comments.
As you can see, if you configure the plug-in right in the Plugin’s settings for SABRE, it is much harder for spambots to register and it might deter some human spammers from joining since they have to also provide an e-mail for e-mail confirmation.
Download SABRE from http://wordpress.org/extend/plugins/sabre/
You might also be interested in reading my post on: WordPress Plugins for reducing automated Comment Spam on Blogs