Usually Spammers don’t have enough time to manually spam a blog, so they hire robot to do the nasty job. And to prevent them, we use tools like Akismet or Spama Karma.
They effectively block spams, but at the same time they kill good comments as well. Ofcource, you can verify the spambox before hitting the delete button, but sometimes it becomes nearly impossible to verifying each one of them.
They only way to kill these robotic spams is to install Captcha (Captcha is an acronym for Completely Automated Public Turing test to tell Computers and Humans Apart).
While there are tons of WordPress Plugin which does the job, I use only Peter’s Custom Anti-Spam plugin because its neat and high customizable.
Installation is fairly simple, just download the plugin, upload it inside your plugin directory and then activate the plugin through your WordPress Admin Panel.
This will automatically insert a captcha above your comments form, but if you want to manually insert the captcha code in your template or change the configuration, you’ll need to edit custom_anti_spam.php. Don’t worry, it isn’t that hard, you’ll find all the details with the file itself.