Comment spam is a fact of life if you have a blog. Using WordPress, you have not only solid built-in tools to prevent comment spam, there are also a wide range of comment spam protection and defense plugins and methods to choose from if you feel you need additional coverage and protection.
There is no “one size fits all” method that will protect your comments; spammers use many tactics. Consider using multiple defenses. Remember spammers change the way they attack so you must keep your choices updated.
1. Akismet – Akismet checks your comments against the Akismet web service to see if they look like spam or not and lets you review the spam it catches under your blog’s “Comments” admin screen.
2. WP-reCAPTCHA – reCAPTCHA is an anti-spam method originating from Carnegie Mellon University which uses CAPTCHAs in a genius way. Instead of randomly generating useless characters which users grow tired of continuosly typing in, risking the possibility that spammers will eventually write sophisticated spam bots which use OCR libraries to read the characters, reCAPTCHA uses a different approach.
reCAPTCHA has earned a very prestigious reputation among the various CAPTCHA systems available and is used by such sites as Facebook, Twitter, StumbleUpon, and a few U.S. Government Websites such as the TV Converter Box Coupon Program Website.
3. WP-SpamFree Anti-Spam – 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.
4. SI CAPTCHA Anti-Spam – This plugin adds CAPTCHA anti-spam methods to WordPress on the comment form, registration form, login, or all. In order to post comments or regiser, users will have to type in the phrase shown on the image. This prevents spam from automated bots. Adds security. Works great with Akismet. Also is fully WPMU and BuddyPress compatible.
The aim of this plugin is to prevent automated attacks (by bots) on the following WordPress actions:Posting comments,Logging in,Registering for a new account,Requesting a lost password
6. NoSpamNX – NoSpamNX automaticly adds additional formfields to your comment form, invisible to the users. If a spambot fills these fields blindly (which 99.9% of all spambots do), the comment will not be saved. You can decide if you want to block these spambots, mark them as spam or put them in moderation queue. Furthermore, you can put common spam-phrases on a blacklist.
7. WP-NOTCAPTCHA – This plugin adds CAPTCHA anti-spam methods to WordPress on the comment form, registration form, or both. In order to post comments, users will have to range icons in right (upright) place. This can help prevent spam from automated bots (100%).
8. Block-Spam-By-Math – This plugin protects registration, login and comment forms from spambots by adding a simple math question to it (“what is 4+3 ?”). This approach should give you 100% anti-spam protection to your sites.
9. Geo Captcha – Geo Captcha uses the GEO-IP-library GeoLite Country by MaxMind to get to know from which country the commentator comes from. If this country is not registered in the whitelist (can be edited in the admin interface) the commentator needs to enter a captcha, which is generated by SecurImage
10. spam-stopper – 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.
You can ask anything, keep it simple and obvious, e.g. Is the sky Green? NO!. Doing this will stop stupid bots from being able to post.
It only shows up when you are logged out, so you dont have to fill it in
11. WP Captcha-Free – WP Captcha-Free blocks automated comment spam without resorting to CAPTCHAs. It does so by validating a hash based on time (and some other parameters) using AJAX when the form is posted. Comments posted via automated means will not have a hash or will have an expired hash and will be rejected. Unlike using a captcha, this does not place any burden on the commenter.
There are some more which i m just listing here as links –