* Get to the System->Preferences->Keyboard menu.
* Select the "Layouts" tab and click on the "Layout Options" button.
* Then select "Key sequence to kill the X server" and enable "Control + Alt + Backspace".
If this doesn't work (e.g. the option is unchecked but the key sequence still works), you can edit your /etc/X11/xorg.conf:
The DontZap option in the /etc/X11/xorg.conf file enables or disables the CTRL-ALT-BACKSPACE shortcut to restart the X-server. In previous Ubuntu releases, this shortcut was enabled by default, however this has changed with 9.04 "Jaunty", as a number of users have complained about accidentally restarting their X-Server, which results in a sudden, unexpected logout which brings to the logon screen.
You can install the dontzap package and use the command dontzap --disable, or you can manually edit your /etc/X11/xorg.conf,
sudo nano /etc/X11/xorg.conf
then type your password, press enter, and use the simple text editor to add a ServerFlags section like this to the start, end, or middle of the file (Ctrl+O to save file):
Option "DontZap" "false"
And then restart the Computer (or log out and back in) to apply the changes. The shortcut should be re-enabled now.
If you want to disable the shortcut, either delete the section (in Jaunty) or change it to this or add this:
Option "DontZap" "true"
And then restart the Computer (or log out and back in) to apply the changes. The shortcut should be disabled now.