Users can choose whether or not multiple submissions by the same user should be allowed in their forms and polls.
You will find the appropriate options in your POWr Editor under Settings > After Form Submission:
Selecting "Allow one submission per page load" will prevent the user from submitting multiple times during the same visit. However, once they refresh the page, or come back to it later, they will be able to submit again.
If you don't want users to be able to submit again after page refresh, you can toggle on "Only 1 submission per user":
This option sets a browser cookie and tracks IP, preventing a user from submitting at a later time as well as preventing multiple submissions made from the same network.