prevent avia_ajax_form submit

Hi,
I’ve generated an custom form in functions.php of my enfold child theme based on avia_ajax_form.
The purpose is to use the styling and frontend validatation – this already works well.
Now I need to implement a custon ajax handler for processing the post data.
The question is: how can I prevent the standart avia_ajax_form ajax submit to use my custom ajax submit implementation?

<form ... class="my-custom-form" ... onsubmit="return false;" ...>
or

$('form.my-custom-form').submit(function(e) {
  e.preventDefault();
});

doesn’t work

Thank’s for any help!
cheers TJ