WPMU DEV LOCK POSTS Plugin
Lock any post across your network from being edited by any user – including site administrators.
WPMU DEV LOCK POSTS Plugin lets a network super admin ‘lock’ or ‘fix’ posts so they can’t be edited by the users of that site – even administrators.
Block post editing
Perfect for stopping school assignments from being edited after submission or adding a post to a network that you don’t want removed.
Lock Posts adds a post status module that’s only available to post or page admins. Easily toggle the status between locked and unlocked.
Please visit official plugin page for more information