If you are allowing the general public, access to your computers (Like for example, in a cyber cafe), then you will be interested in restricting the users from shutting down or rebooting your Linux machine. The following are the steps needed to accomplish this:
Disable access through the Action Menu (Applicable to GNOME desktop)Run the gconf-editor program on the GNOME desktop,
and check off the entry for /apps/gnome-session/options/logout_prompt.
Disable shutdown and reboot commands at the login screen
The gdm daemon is responsible for managing the login screen in X. Edit the /etc/X11/gdm/gdm.conf file, and set the 'SystemMenu' directive to 'false'.
Note: The gdm.conf file is a liberally commented file which contains a lot of configuration parameters which can be changed to modify how the system logs in for an X session. For example if you want the system to log in automatically to a users account after an interval of logout period then you set the 'TimedLoginEnable' parameter to true.
Prevent users from executing these commands in the console
Rename the reboot, poweroff, and halt files under the /etc/security/console.apps/ directory.And finally ...
Disable the Ctrl+Alt+Del key combination
Comment out the following line in the /etc/inittab file:
# FILE: /etc/inittab
# ca::ctrlaltdel:/sbin/shutdown -t3 -r now
This will disable the Ctrl+Alt+Del key sequence . Now only root can power off or reboot the machine.
Give selective superuser powers to users.