From
http://www.linuxsa.org.au/mailing-list/1999-03/377.html
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Check out /etc/inittab
About a page down there is an entry:
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
just put a # in front of the ca::ctrlaltdel... line, and you'll have
done it.
You'll need to tell it to reload its files though, so once you're back
at the command line type `telinit q`