This is due to the fact that “DontZap” is no longer an option in the X server and has become an option in XKB instead.
* 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".
* Launch "systemsettings"
* 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".
Using the command line
You can type the following command to enable Zapping immediately.
setxkbmap -option terminate:ctrl_alt_bksp
If you're happy with the new behaviour you can add that command to your ~/.xinitrc in order to make the change permanent.
You can add the following line in /usr/share/hal/fdi/policy/10osvendor/10-x11-input.fdi (inside the <match key="info.capabilities" contains="input.keys"> section):
<merge key="input.xkb.options" type="string">terminate:ctrl_alt_bksp</merge>