1. Akismet Plug-in:
Akismet is a great plug-in to prevent your blog from comment and track-back spam. Akismet is now embedded in WordPress core and come with WordPress by default. It does a good job of picking up spam. Akismat uses a centralized database of spammer IPs and can recognize 99% + spam successfully. Akismet is a must for every blogger. To make Akismet work at your blog you need its API key. Akismet API key can be aquired by registering at WordPress.com.
2. reCaptcha Plug-in:
Large numbers of sites uses Captcha to protect their sites form spamming. Spam bots can’t read captcha images. You might have notice that big sites like Facebook, Twitter, Digg and StumbleUpon uses captcha. CAPTCHA is (an image containing some letters that are designed so only humans can read them but spam bots can’t read them). You can easily integrate Captcha images in your comment form using reCaptcha WordPress plugin.
3. Ask Math Question.
Ask Math Question is a plug-in which can ask simple math questions to commentators during comment submission. Humans can read question and give answer easily. Plug-in name is “Math Comment Spam Protection” you can download it from link below.
4. Stop Trackback Volition:
Trackback spam has also skyrocketed and many spammers are misusing trackbacks. You can use “simple Trackback Volition” plugin for protection against trackback spam. This plug-in checks IP address of the sender of trackback. This plug-in also retrieves the web page located at the URL included in the trackback, so trackback spammers won’t use bots running on infected machines. If the page is not linking to your blog the trackback link will be considered as spam. This plug-in quickly block illegitimate trackbacks.