Hi. I don't know of a way that the window that comes up at logout can be disabled. I kind of doubt that's possible without hacking the OS. Its not even a good idea really since that window provides an out in the event the Logout option was chosen accidentally or the shortcut keys were pressed.
As for the checkbox, that setting changed slightly with Mountain Lion. It was a boolean value in Lion, but now its a string value (which I thought was odd) Its still stored in the same com.apple.loginwindow plist at the user level and is located under the TALLogoutSavesState key. it doesn't appear its possible to grey out the checkbox though. You can apply the setting via MCX at every login, but users will still be able to check it back on at every logout.
To untick the checkbox:
defaults write ~/Library/Preferences/com.apple.loginwindow TALLogoutSavesState 0
To tick it back on:
defaults write ~/Library/Preferences/com.apple.loginwindow TALLogoutSavesState 1
Those could be easily converted into MCX settings:
Definition:
User Level: Allowed
User Managed Level: Not Allowed
Computer Level: Not Allowed
Domain: com.apple.loginwindow
Key Name: TALLogoutSavesState
Key Type: string
General:
Apply Setting to: User Level at Every Login
Value: 0
