If you want to disable CTFL-ALT-DEL from shutting down your server use the following procedure.
To disable the reboot action taken by pressing the Ctrl+Alt+Delete key combination, comment out the following line in the file /etc/event.d/control-alt-delete.
sudo vi /etc/event.d/control-alt-delete
Change the following line
exec /sbin/shutdown -r now "Control-Alt-Delete pressed"
#exec /sbin/shutdown -r now "Control-Alt-Delete pressed"
Save and exit the file