It is pretty easy to do, for anyone else that may be wondering how to, edit the viewticket template.Admin CP > Templates > Manage Templates > default > Tickets > viewticket
1) Edit the viewticket template and add an if else clause to remove the “Post Reply” button if the ticket status is closed.
2) Staff CP > Groups > Guest/Register > Edit > Permissions: Tickets > Can Change Ticket Properties > No
Anything you want to disable if closed, just wrap in this:
<{if $tvstatus != "Closed"}>
<{/if}>
And if the ticket status is then set to closed, it will automatically stop displaying anything wrapped in that conditional.
I used it to remove the ticket update section and reply button from closed tickets, and to add a line of text that displays:
In addition, if you add a mail parser rule that simply ignores replies to tickets with closed status, and sends an auto reply with a similar message, you have permanently closed the tickets (I think!)
If there is a better way to do this, let me know.