This article explains how to protect your WordPress blog from spam. There are several measures that you can take to prevent malicious comment getting into your site.
Keep WordPress Up-to-date
Make sure that your WordPress application is updated to the latest stable version. Each WordPress update includes a variety of patches, security fixes, and new features. Often, there are anti-spam measures added to the core to prevent spam from getting in.
Install and Activate Akismet
Akismet is a plugin by Automattic that checks each comment you get into your site against an enormous database of spam patterns. Then it filters out spam automatically without bothering your users at all. In order to use Akismet, you need a key that you get for free from the official Akismet site.
Install a Visual Verification
If spam keeps getting through, you may want to install a CAPTCHA plugin. A good choice is a WordPress Captcha plugin.