Currently there is no restriction on how many times a user can submit a form. The only workaround would be to add text to your form telling the user to only submit the form one time (basically the honor system). Also if a user does submit it multiple times you can always delete the duplicate data from the view response tab.
The only way to know who filled out a form is to put a required field on the form to collect the user's name or email address.
Hope that helps.
FormsCentral product manager
I know this is an old question, but has this been resolved yet?
I am recommending to my company to subscribe to this service, but one thing we would really like is to prevent multiple submissions if at all possible.
Would be good to hear if this is possible yet?
This hasn't bubbled up yet to be a top request. It is something we want to provide in the future but I can't say when.
I would definately encourage this feature to be provided. Only allowing a single submission allows FormsCentral to be used for truly accurate data gathering. Sadly relying on the honor system isn't something which (in my experience) works very well.
Is it as simple ad embedding a cookie?
I would also say this would be highly used function to this application. I know it would be something we could use repeatedly.