Admin settings GDM

Several to disable options in GDM and gnome:

You need to change gdm's config to remove the menu options.
Open /etc/X11/gdm/gdm.conf in a text editor and find the [greeter]
section. Make sure there is a line like this:

[greeter]
...
SystemMenu=false

This removes the shutdown/reboot options from the log-in screen and the
log-out dialog. Alternatively run gdmconfig and provide the root
password, then uncheck the 'Show action menu' option on the security
tab.

To prevent users opening a terminal and running poweroff or reboot,
delete or move these files:

/etc/security/console.apps/poweroff
/etc/security/console.apps/reboot

If your machines use ACPI, you will also need to disable ACPI-driven
shutdowns from the power button.

On more to go now, to prevent someone switching to a virtual console and
hitting ctrl-alt-del, we need to edit inittab. Find the line that looks
like this:

# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now

And change it to read:

# Disallow CTRL-ALT-DELETE
ca::ctrlaltdel:/bin/echo "ctrl-alt-delete has been disabled"

(You can use /bin/true, or /usr/bin/logger etc. here instead if you wish
- just watch out if someone tries ctrl-alt-del before /usr is mounted :)