I think there is no option to avoid multiple votings. Adobe has not implemented this funktion.
Formstack is as alternative to Formcentral. The Fieldoption there is called - Unique.
Unique allows you to restrict submissions of the same value in this field. For example, you can allow only one submission for a particular email address or physical address.
That is the option i was looking too.
Formcentral is useless for me in this way.