JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
If you wish to limit your users to vote per unique email. That is possible by enabling your form limits.
Here's a guide about it:
To limit submission per unique answer in your form fields (like your email field), then you can follow this guide: http://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question
To limit your submission per browser OR by unique IP, you can follow this one: http://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form
I hope that helps.