Captcha

Turn on the built-in math captcha on the Settings tab. Each challenge is bound to that form, works once, and is signed on the server.

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Enable math captcha, then open the public form. Capture the challenge above submit.

Replace this box: save a PNG at public/docs-screenshots/access-math-captcha.png

You can also insert a Google reCAPTCHA block. See reCAPTCHA.