You need to prohibit users to use Ctrl+Alt+Del to reboot the system. Complete the following line in /etc/inittab

ca:12345:________:/bin/echo “Reboot not allowed”

Answer: ctrlaltdel

Explanation/Reference:
/etc/inittab consists of many lines with the following format:
id:runlevel:action:process

id is just an unique identifier, with no higher meaning
runlevel defines for what runlevels the line is valid (empty means all runlevels)
action is one of the following: boot bootwait boot ctrlaltdel initdefault once ondemand powerfail sysinit boot

bootwait respawn wait

Example:
ca:12345:ctrlaltdel:/sbin/shutdown -r -t 4 now