When a new comment, trackback, or pingback comes to your blog it is submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down. There’s no maintenance or upgrading needed and it comes with WordPress.
DNS-anti-spam is a plugin to protect your weblog from spam-comment. It checks the IP address the comment is posted from against a few blacklists, and checks the content of the comment against a huge list of known spammer websites
Spam filer works by checking a user behavior via webserver log file
An extremely powerful WordPress anti-spam plugin that eliminates blog comment spam, including trackback and pingback spam. Finally, you can enjoy a spam-free WordPress blog! Includes spam-free contact form feature as well.
Selectively close comments on your blog to help reduce spam. It comes with an administration page and is fully configurable.
The whole idea to this plugin is to keep spammer robots from posting on your blog, reducing the space taken by spam messages in the database, and preventing your blog from being a spam magnet.
Challenge restricts comment spam by throwing the commenter a challenge. The default challenge is an addition and multiplication math question. The challenge and the answer can also be user-defined. They can be of anything including PHP-evaluatable expressions.
Use this plugin to bounce back referrer spam attempts. It requires no upfront configuration and no mod-rewrite. It is like the strong silent bouncer at your favorite club and it works well.
This plugin implements spam filtering using Naive Bayesian classifier. This plugin will automatically classify new comments as legitimate or spam based on past decision done by you. It requires modification to a few WordPress core files so it may not be suited for completely new users.
This plugin provides protection against email harvesters by taking email addresses and mailto links on your blog and converting their text to numeric character references.
ImgProtectr will take any text you pass to the imgProtectr() template tag and put an image with the specified text in it’s place. This is primarily intended for use in templates where you would have an email address displayed.