How do I restrict customers from editing the ticket properties after a ticket is submitted?

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.