In addition to CAPTCHA, you can password-protect your form to prevent spam in your form. Also, if you have made your form URL public, the password field will prevent anyone from filling your public form unless they fill the correct password.
To add a password field to your form, open the sidebar inside Google Sheet and expand the
Advanced Settings section. Type a password in the "Form Password" field and save your changes.
The form does client-side verification of the password.