Spam Comments are always a problem for bloggers. I also faced this problem and the biggest problem comes from auto 
comment posting. You may get hundreds of spam comments per day and you may spend lot of time to manage them. These days there are many software which can auto submit comment to WordPress based blogs and getting rid of them is difficult task.
To prevent your WordPress based blog there are many great free plug-in which can deduct spam comment and can move them to spam comment or can delete spam those comments. I am listing such few plug-in which can protect our blog from spam comment.
5 Anti-Spam Comment Plugins
Akismet– Akismet is most popular spam protection plug-in for WordPress. 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. To install Akismet you require WordPress API which you can get by registering on wordpress.com. There is one problem I faced in Akismet that it can’t stop auto submit comment yes it recognize them as spam comment.
WP-SpamFree Anti-Spam – An extremely powerful WordPress anti-spam plugin that eliminates blog comment spam, including trackback and pingback spam.WP-SpamFree eliminates automated blog comment spam from bots, including trackback and pingback spam.It works like a firewall to ensure that your commenters are in fact, human. No CAPTCHA’s, challenge questions or other inconvenience to site visitors – it works silently in the background.
Math Comment Spam Protection – Math Comment Spam Protection asks the visitor making the comment to answer a simple math question. This is intended to prove that the visitor is a human being and not a spam robot.
Example of such question: Sum of 9 + 9 ?
This plugin does not require JavaScript or cookies. It uses a special encryption function; the result of the question is being passed via a hidden field. The result is being compared with the value entered by the visitor – after encrypring this value as well.
Yawasp – Most anti-comment-spam-plugins focus on user interaction, e.g. captcha or math comment spam protection. Some use JavaScript and/or Sessions and check each comment against common spam phrases. But in most cases you still have to manage some spam. Yawasp focuses on handling comment-spam without these measures. It replaces the names of the comment form fields with random names, protecting your blog from spambots that aim at the default comment field names. Furthermore it adds a blank field, hidden from the user, that needs to be left empty. If a spambot fills this field blindly, the comment will not be saved. In addition, the random field names are changed every 24 hours, giving a spambot no chance to adapt to the comment form.
TanTanNoodles Simple Spam Filter – A simple pre-filter to weed out the most obvious comment spam (about 90% of all spam). Legitimate comments that get blocked (either by this plugin or by Akismet) can be presented with a captcha to confirm that the comment is legitimate. Comments that don’t pass the captcha will be immediately discarded.
