If you are using gdm, you should be able to remove the shutdown option at the login screen by opening /etc/X11/gdm/gdm.conf and changing:
You can edit the sudoers file to prevent anyone but root from shutting down then machine by typing visudo as root at the command prompt. Look for lines like the following:
If you remove SHUTDOWN from the %users line, regular users will not be allowed to shut down or reboot the machine.
Hope this helps.