Whoa. David was suggesting that you change the privileges on the shutdown command in /sbin so that only root (or a member of a group with equivalence) can execute it. You would make the changes via chmod or the file-manager of choice in your distribution. If you did this, in *theory* you would have to "sudo shutdown -h now" and you would be prompted for the root password.
However, try this instead:
Step1:
Open /etc/X11/gdm/gdm.conf in a text editor and find the [greeter] section. Make sure that there is a line which says SystemMenu = false. This line will ensure that the gdm login screen will not have the option for shutting down the system etc.
Step 2:
If you have a laptop, or an acpi system on your computer, then go to /etc/acpi and disable the power button, so that, when someone presses the power button, the system does not shutdown. You can disable this easily by doing chmod 000 /etc/acpi/powerbtn.sh
Step 3:
Now edit /etc/inittab
and find the lines that say:
#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” .
This will effectively prevent users from changing to a console screen and using ctrl+alt+delete to shutdown the system.
Step 4:
Execute the following commands:
sudo chgrp admin /sbin/halt /sbin/shutdown
sudo chmod 550 /sbin/halt /sbin/shutdown
Step 5:
Use the Configuration Editor (Applications -> System Tools -> Configuration Editor) to edit the preference apps->gnome-session->options->logout_option to “logout” instead of “shutdown”.
Now only a superuser (Admin) can shutdown the system, using the command:
$sudo shutdown -t3 -r now