Let's say that you have set up a multi-seat system for use in a school or library, allowing GDM to launch multiple X sessions to run simultaneously with different users/keyboards/monitors.
By default in Debian/Ubuntu in Gnome, you don't have to be root to shutdown or reboot. But this means any user can choose "reboot" or "shutdown" and kick off the other three users.
You have blocked physical access to the server so they can't simply push the power or reset buttons.
What is the correct way of disabling the "shutdown" and "reboot" functionality which is exposed to regular users through GDM/Gnome/whatever window manager you're using?