Forms for Confluence makes use of Confluences built in spam prevention mechanisms through the use of a CAPTCHA. For spam prevention to work, you will first need to have spam prevention turned on through your administration console. Please see Atlassian's guide on configuring Captcha for Spam prevention.
Starting from Forms for Confluence 7.0.0 release you can enable or disable captcha when you insert or edit the Forms macro. The "Advanced Option" tab contains the option along with other relevant configurations.
The "Spam Prevention" section won't appear if spam prevention is turned off from the Confluence spam prevention settings.