This isn't something that FormsCentral supports.
The only idea I have is to embed the form into your own website where you set up those controls on the page you embed the form into (such as a login for the page where each user can only log in once), after the form is submitted the redirect URL (a feature in FormsCentral) could take the user out of that page so that they have to sign in again but would then be blocked from signing in a second time (by a system that you've set up).
The other method you can look at was described in this forum thread, it does not fully enforce a single submission but does restrict (this is using PDF):http://forums.adobe.com/message/5929599#5929599
Thank you for your quick response!
This feature is definitly on my wishlist for FormsCentral!
Other systems provide unique links (e.g.).
A login procedure keeps a lot of participants from answering questionnaires...