How can I block someone from being able to fill out any of my forms?

How can I block someone from being able to fill out any of my forms?

Though it is not possible to block a specific user from filling out your forms, it is possible to set up unique submissions for forms:

https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form

Or a unique question:

https://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question

This way, they would only be able to fill out the form once.  This would also apply to other users attempting to fill out the form.