The Jetpack Contact Form captcha replacement extension is completely  FREE and it’s integrated in WPBruiser’s core code. At the click of a button all your forms built with Jetpack Contact Form module will be protected by WPBruiser.

Installing WPBruiser – Jetpack Contact Form Extension

• In case you don’t already have WPBruiser installed, go to Dashboard -> Plugins -> Add New
• Search for WPBruiser in the provided box , and click “Install Now”
• After the installation finishes, click “Activate” and you’re ready to configure WPBruiser
• Go to Dashboard -> WPBruiser -> Contact Forms
• Activate protection for Jetpack Contact Form

Download WPBruiser from WordPress.org