While there is a notice at the top that says:
Not open for further replies.
for a person browsing the thread, after reaching the end, its not so obvious why there is no reply box. The "No Permission to reply" is not very well placed (to the right and out of your immediate eyeview) and worse, it does not explain why you don't have the permission in the first place.
I suggest displaying a visible closed notice at the bottom of the showthread page also. ANd preferably in eye-catching colours (eg red)