Disable and remove the Shut Down command
User Configuration\AdministrativeTemplates\Start Menu & Taskbar
Prevents users from shutting down or restarting Windows.
This policy removes the Shut Down option from the Start menu and disables the Shut Down button on the Windows Security dialog box that appears when you press CTRL+ALT+DEL.
This policy prevents users from using the Windows user interface to shut down the system, although it does not prevent them from running programs that shut down Windows.
If you disable this policy or do not configure it, the Shut Down menu option appears, and the Shut Down button is enabled.