You can enable/disable CAPTCHA on a per-form basis.
Find and select your desired form.
Click Edit Draft (if the form is already approved, click Create Draft).
Click Form Settings, then Settings.
Open the CAPTCHA drop-down and select Enabled.
Click Finish.
Click Approve and Close.
It may take a few minutes for changes to this list to propagate from Google.
Standardized text, including links to Google’s Privacy Policy and Terms of Service, will be added to your CAPTCHA-enabled form(s).