How do I stop a respondent filling out the form more than once?

You can turn on or off Unique submissions option in your form settings: https://www.jotform.com/build/90448074337864/settings

This guide should help as well: https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form

Let us know if you have any further questions.

Thank you!