JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Unfortunately, we currently do not have an option to do that.
However, it is possible to do this via your jotform's source code. Please see this thread here http://www.jotform.com/answers/355370-How-to-stop-form-submitting-when-enter-is-pressed for a workaround provided by one of our supporters who explains how to do that.
Or you could also add a test code field so that accidental submissions using the "Enter" key would be avoided.
Or you could also add the Captcha field and make it required so that accidental submissions can be avoided.
I hope this helps.