First, your Wagn must be configured with ReCaptcha keys. If your Wagn is hosted at wagn.org, then your keys are already configured for you. If not, please see the "key configuration" section below.
To turn captcha cards on and off, edit the captcha setting in a card's rules.
The captcha setting is a toggle; when it's on, every card that a user can create, edit, comment on, or delete without signing in will require a captcha. Naturally, captchas are never required for signed-in users.
If you are hosting your own Wagn, you will need to register for ReCaptcha keys and add them to config/application.rb, like so:
config.recaptcha_public_key = '6LdhRssSAAAAAFfLt1Wkw43hoaA8RTIgso9-tvtc'
config.recaptcha_private_key = '6LdhRssSAAAAAGwzl069pJQBdmzCZigm1nV-dmqK'