In the meantime i found a solution.
I've added a section to the page of the mpform.
The extra section is a code module where i've wrote a script that remove all Non-ASCII characters from my large textfield (in my case field #7).
The script is executed when you click on the send button.
echo "<script>
$('.mpform_submit').on('click', function () {
removeInvalidChars();
});
function removeInvalidChars() {
var str = $('#field7').val();
str = str.replace(/[^\x20-\x7E]/g, '');
$('#field7').val(str);
}
</script>";