How can I block a user from responding to the form 2 times through the validation of their email?

I do not want to allow the same user to respond to the form 2 times using the same email.

It is possible by setting the email address field as the unique question field in the form settings. Please refer to the guide below that can help you to achieve this requirement.

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

Hope this information helps!