How do I prevent someone from registering more than once?

Please take a look on the following guides that will help you setting up unique submissions: 

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

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

I hope this helps.