I have written a WordPress Plugin: 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.
Download the latest version (ZIP).
If you have a question about the plugin please go to the plugin’s homepage.