Block Top Spammers displays a list of your top spammers’ IP addresses, based on all comments in your database that are marked as spam. It also generates a blacklist for your
.htaccess file to block those spammers from your website entirely, thus taking load off the server. – You will need another plugin (like Akismet) to identify the spam.
PHP 5 and Apache required.
Free Download: Download this wp-plugin