Restrict Registration

“Restrict Registration” is a plugin for WordPress that allows you to control
who is registering on your blog. This plugin uses simple blacklists and
whitelists to define who can register for your blog, and who is restricted. Use
simple literal matching, flexible asterisk wildcard matching, or powerful
regular expression matching: it’s your choice! The options page is clear and
easy to use. Set your lists, and you’re done!

This plugin was inspired by WP-Deadbolt, but has been completely rewritten and
redesigned, and several new features have been added.