Don’t think this as a flaw to the GNU/Linux system, but can be helpful in many situations. This part plays a vital role in the security of a system. In GNU/Linux system pressing Ctrl+Alt+Del can cause a shutdwon/reboot by default. As I said this isn’t a bad configuration and most of us may be using this as a shortcut. Moreover it is pretty important if you don’t have the best physical security to the machine. Commenting out the following line/lines will disable the possibility of using the Control-Alt-Delete command to shutdown your computer.

In debian based distros
nano /etc/init/control-alt-delete.conf

   Comment out the lines in the file to look like
   # control-alt-delete – emergency keypress handling
  #
   # This task is run whenever the Control-Alt-Delete key combination is
  # pressed, and performs a safe reboot of the machine.

  description     “emergency keypress handling”
  #author         “Scott James Remnant <scott@netsplit.com>”

  #start on control-alt-delete

  #task
  #exec shutdown -r now “Control-Alt-Delete pressed”

In redhat based distros
nano /etc/inittab

Comment the following line

    ca::ctrlaltdel:/sbin/shutdown -t3 -r now

to look like

    #ca::ctrlaltdel:/sbin/shutdown -t3 -r now
Save the file and logout.