Does your system REQUIRE payment before submitting?

Do your forms force the user to make payment prior to completing/submitting the form? Or do the forms just allow submissions without any required payment?

This will depend on how your form works, for example, if you're hiding the payment field with conditions then payment will not be required, if the payment field has not been set as required then the form can be submitted without payment as well. 

It will also be different depending on the payment integration you're using, if you're using PayPal, for example, your users will be redirected to the PayPal payment page and here if they close the window the payment will not be completed, but the form will be submitted. 

I will suggest you to set your payment field as required, to do it click on the down arrow icon and select the option "Required": 

With PayPal there is not much that you can do to avoid your users to simply close the window without completing the payment, but you may set the form to send you an email each time this happens, here is a guide that will help you with that: How-to-Setup-Incomplete-Payment-Notification-E-Mail

If you have any question, please let us know.