The above is correct in that the power off (holding for 4 seconds) is usually hard wired into the hardware, either way it would not stop a determined user from simply pulling the power cable, the instructions above would limit the users who can run shutdown. The CTRL+ALT+DEL combination is caught by the init process, the following line
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
in /etc/inittab traps the CTRL+ALT+DEL press and determines what the OS should do with it.