[BirdOPrey5]

That would be a tough one to add an option to every thread, it would require modifying the database and templates and custom plugins.

If however it's something you will seldom use you could make a new plugin-

Admin CP -> Plugin Manager -> Add New Plugin

Product: vbulletin
hook: postbit_display_complete
title: disable edit in thread
execution order: 5

PHP Code:

if ($thread['threadid'] == 1000)
   $post['editlink'] = "";

Set Active to YES and SAVE.

Change 1000 to be the threadid of whatever thread you want to disable edits on.

That will disable edits for everyone.

If you want to allow mods and admins to edit:

if ($thread['threadid'] == 1000 AND !is_member_of($vbulletin->userinfo, 5, 6, 7))
   $post['editlink'] = "";

Where 5, 6, and 7 are mod and admin usergroups.