1. Edit /etc/inittab. Any text editors will do- I like nano but vi works just as well.
2. Search for "CTRL-ALT-DELETE" or "ctrlaltdel"
3. Comment out the line "ca::ctrlaltdel:/sbin/shutdown -t3 -r now" with a # symbol.

It should look like:

# Trap CTRL-ALT-DELETE

# ca::ctrlaltdel:/sbin/shutdown -t3 -r now

4. Save and exit the file.
5. To make this take effect without a reboot, run the command:

init q

References