How can I restrict submissions to one per user?

It's not possible to restrict submissions to one per user. Forms do not require users to be logged in so there is no way of definitively knowing if someone is submitting a form twice. There are some ways you can process your submission data to remove duplicates.

First export your submission data to a spreadsheet application. Then you can filter your submission data by IP address or a unique field (such as email) to remove duplicate submissions.