In Ubuntu; Hitting CTRL-ALT-DELETE on most Linux distributions will start the soft reboot process.
Ubuntu: This operation is controlled via a special file located at /etc/event.d/control-alt-delete directory. Simply open this file using a text editor:
sudo vi /etc/event.d/control-alt-delete
sudo gedit /etc/event.d/control-alt-delete
Find line that read as follows:
exec /sbin/shutdown -r now "Control-Alt-Delete pressed"
delete configuration line or comment out line by prefixing # symbol:
#exec /sbin/shutdown -r now "Control-Alt-Delete pressed"