Q. How do I delete control-alt-delete emergency keypress handling to improve security of my Ubuntu Linux server?

A. When you hit the Control-Alt-Delete key combination system get rebooted. 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
OR
$ 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 # symob:
#exec /sbin/shutdown -r now "Control-Alt-Delete pressed"
Save and close the file.