It used to be ctrl+alt+backspace to kill x server, but for some unknown reason, it was decided to disable that combination and use something else (I think it is right alt + printscreen +k).
So, how to enable ctrl+alt+backspace?
I have seen it can be done with installing dontzap, but I know there is a way to set that combination from system setup.
Gnome:
Open the keyboard preferences (Search for keyboard in Dash), go to the Layout tab and press Options. Open Key sequece to kill the X server and toggle Control + Alt + Backspace
KDE:
I don't run KDE so I can't verify it or make a pretty screenshot, but according to the Ubuntu Wiki, you do as follows:
KDE 4.1 - KDE 4.5
Select "Regional & Language"
Select "Keyboard Layout".
Click on "Enable keyboard layouts" (in the Layout tab).
Select the "Advanced" tab. Then select "Key sequence to kill the X server" and enable "Control + Alt + Backspace".
KDE 4.6+
Select Advanced Tab
Click on Configure Keyboard Options
If you are looking for a solution which is independent of Gnome/KDE/Fluxbox or any Desktop Environment or Window Manager, try the following. This command is part of the X Window System.
From https://wiki.ubuntu.com/X/Config/DontZap#Using_the_command_line
Using the command line
You can type the following command to enable Zapping immediately.
setxkbmap -option terminate:ctrl_alt_bkspIf you're happy with the new behaviour you can add that command to your
~/.xinitrcin order to make the change permanent.
I have provided more information in my answer to 10.10 How can I enable Ctrl+Alt+Backspace?
Navigate to the keyboard preferences, choose layout and then options. You will find the ctrl+alt+backspace there, just tick it and your good to go
