Hi,
go to your form > advanced >more options > form pieces > before form > add the following code in there :
echo '
<script>
jQuery(document).ready(function(){
jQuery("#bfSubmitButton").on("click", function(){
jQuery("#bfSubmitButton").attr("disabled", true);
});
});
</script>
';
This will disable Submit button after it's clicked for the first time.
Regards,
Tihana