Ontraform submission - limit to 1 submission per email address

I’ve just published a form and I see multiple submissions from the same email address (with different names and contact number).

There is no way to limit the form submission to 1 per email address... the Support team mentioned that this is only possible if the form was included in a campaign - that way I'd be able to use a trigger setting. 

I hope that the team can look into this ASAP. Thanks!