Sir-Uli

This plugin provides a possibility to block comments based on words of a predefined list. The built-in lists of wordpress only allow to send comments to the moderation- or to the spam-queue but not to block them completely. This is remedied with this plugin, which allows to block comments based on a definable list.
Attention #1: Use wordlist with care, there is no logging of the blocked comments!
Attention #2: It will also match inside words, so “press” will match “WordPress” as well!

WU – Block comments

This plugin provides a possibility to block comments based on words of a predefined list.

Daniel Davis

No-captcha Spam Block is a WordPress plugin that automatically blocks comments it thinks are spam. It works tirelessly, day and night, applying various checks to each comment that comes in and giving it a spam score – any score too high and the comment goes straight to the spam bucket in disgrace.
<ul>
<li>No third-party registration</li>
<li>No JavaScript required</li>
<li>Best of all, no captchas</li>
</ul>
There are no visible changes to your website and you don’t need to set anything up – just install, activate and go!

No-captcha Spam Block

Dramatically reduce comment spam on your blog without using a captcha.

WU – Block comments vs No-captcha Spam Block