In order to use this functionality you must have the WordPress Akismet Plugin installed and configured. When you have done this there is nothing more to do. The contact form plugin will automatically detect Akismet and ask it to parse all submitted forms. Any spam, will be logged in the ‘Comments’ section of your WordPress admin screen and no email will be sent to you.
You can keep an eye on your spam very easily because the spam will be logged as a spam comment for the page that you have put the contact form on.
You can test it out by entering ‘viagra-test-123’ in the ‘Name’ field of the contact form. This text will always be marked as spam by Akismet.
If for any reason you do not want your contact form to be scanned add this to your theme’s functions.php file: