Comment blacklisting works well, but what if you actually want to stop someone from even reading your blog? This plugin will take care of that for you.
It uses .htaccess file modifications to deny access to your blog from particular IP addresses (Deny from) and then follows up with an ErrorDocument 403 redirect to a URL of your choice. It doesn’t hook into WordPress, which makes it a lot faster due to no database or PHP calls.

WP-BlockYou

A plugin that can be used to block or "ban" users via their IP address from viewing your blog, and then redirecting them to a URL of your ch …

w3prodigy

Returns ‘Page Not Found’ 404 error message for IP’s visiting your blog specified in the IP Ban option on the Discussion Options page.

IP Ban

Returns 'Page Not Found' 404 error message for IP's visiting your blog specified in the IP Ban option on the Discussion Options page.

WP-BlockYou vs IP Ban