5.2. Disabling Rebooting Using Ctrl+Alt+Del

The action that happens in response to pressing Ctrl+Alt+Del at console is specified in the /etc/init/control-alt-delete.conf file. By default, the shutdown utility with the -r option is used to shutdown and reboot the system.
To disable this action, create an overriding configuration file that specifies the exec true command, which does nothing. To do that, run the following command as root:
~]# echo "exec true" >> /etc/init/control-alt-delete.override