Spam is not limited to just comments. If you leave your WordPress blog open to new user registration, you could be hit by a wave of spam bots or rather nefarious individuals registering with hopes that you’ll give them a chance to post spam on your blog.
First of all, do you really need open registration? If not, uncheck “Anyone can register” from the Settings area of your Dashboard.
So, what if you need open registration? First, stop the bad bots from even visiting your blog with Bad Behavior.
Now, you could certainly use a CAPTCHA on your registration form, but I wouldn’t bother with that. Several CAPTCHA have been broken by a simple program, and they just aren’t accessible. Instead, use Ban Hammer, which compares registration email addresses with your comment blacklist (just add them if you notice a trend) and the collective blacklist at Stop Forum Spam.
Like comment spam, registration spam will be a constant battle, but Bad Behavior and Ban Hammer should at least make it easier on you.
Following are some plugins which are also effective against SPAM Registrations :
1. Sabre :If you own a WordPress powered blog where users can register freely and see a lot of fake registrations, this plugin can stop fake user registration by bots. It can add image verification or math test to registration process among other measures to make sure fake users are not created.
3. Stop Spammer Registrations Plugin : In all the plugin uses 15 different strategies to block spammers. Eliminates 99% of spam registrations and comments. Checks all attempts to leave spam against StopForumSpam.com, Project Honeypot, BotScout, DNSBL lists such as Spamhaus.org spammers.
4. Wang Guard : Check your registered users against WangGuard web service to avoid Sploggers, is fully WordPress,WordPress MU ,BuddyPress and bbPress 2.0 compatible. WangGuard Always will be free for personal use, this means that whenever you have less than 500 daily registrations or you make less than $200/month, you never have to pay anything for WangGuard.
5. User Spam Remover : User Spam Remover is a plugin for WordPress that automatically removes spam user registrations and other old, never-used user accounts. It also blocks the notification e-mail that WordPress normally sends to the administrator whenever a new user registers (annoying when that registration is spam!) and logs it instead.