CM Anonymous User Posting - Captcha Support
Protecting from Spam - Human verification with reCaptcha
The Anonymous User Posting Add-on comes with reCaptcha support so that you can keep your website spam-free! You can find the URLs to acquire captchas and choose your Captcha settings under the Anonymous Posting tab in the plugin settings.
You need to acquire your API keys for reCaptcha service from Google and enter them into the plugin settings before the Captcha box will work. After entering the API keys, an anonymous user will be prompted to fill in the Captcha when they attempt to post on your Q&A forum.