Comment 6 for bug 792242

It would be nice if it acted like the old gdm of Lucid (?) and required admin password to power off or reboot if other users were logged in, combined in a nice pop up explaining that hang on, someone else is logged in here.

you can make a file like /etc/polkit-1/localauthority/50-local.d/disable-shutdown.pkla
"""
[Disable Shutdown, etc.]
Identity=unix-user:*
Action=org.freedesktop.login1.reboot-multiple-sessions;org.freedesktop.login1.power-off-multiple-sessions;org.freedesktop.login1.suspend-multiple-sessions;org.freedesktop.login1.hibernate-multiple-sessions
ResultAny=no
ResultInactive=no
ResultActive=no
"""

to grey out the buttons in that case, but I've found our students tend to not understand why things are greyed out and pull the power instead of asking. And then once again after reboot when iowait sucks (because the RAID was trying to rebuild for the nth time... sigh)

thanks,
Hamish