How to Add a CAPTCHA to your Form
You can enable Google CAPTCHA for your form to prevent spam entries in your form. When people fill out your form, they'll be required to solve a little puzzle - like identifying cars on the street - and the submission will be allowed only when they prove that they aren't a robot.
To add a CAPTCHA field to your form, open the sidebar inside Google Sheet and expand the
Advanced Settings section. Turn on the option that says "Include CAPTCHA" and save your changes.
Adding a CAPTCHA to your form will also likely prevent visitors from making multiple entries in quick succession since a different CAPTCHA would be served for form request.
The CAPTCHA in forms is powered by the Google reCAPTCHA project.