You can use the below-displayed code to restrict the customer from editing the ticket fields after a ticket is submitted.
Please navigate to Admin--> Channels > Portals -->Customize portal-->Layout & pages-->Portal pages-->paste the below code under the Ticket details section,
<script>
jQuery('#portal_ticket_form .controls').each(function() {
jQuery(this).children().attr('disabled','disabled');
});
jQuery('#helpdesk_ticket_submit').attr('disabled','disabled')
</script>
This would be possible only from the Estate plan onwards in Freshdesk.