This plugin automatically creates a CAPTCHA box for your comment forms, which can reduce the number of spam comments you receive. To activate this plugin, go to the Plugins menu, find SI CAPTCHA Anti-Spam, and click Activate. To change the settings, go to the menu Plugins > SI Captcha Options.
Settings
The default settings are fine for most websites. However, there are some options you may find useful. For example, you can change the CAPTCHA difficulty, or require CAPTCHA for your registered users. If you are using this plugin, we recommend using the Akismet plugin to further identify and filter spam comments.
For more information, see the plugin page(s):
http://wordpress.org/extend/plugins/si-captcha-for-wordpress/