Translations:System Settings/Keyboard/4/en
Under tab, make sure is enabled. Then look in the list below for "Key sequence to kill the X server". Under it, you should be able to check . Once done, click on .
Under Advanced tab, make sure Configure keyboard options is enabled. Then look in the list below for "Key sequence to kill the X server". Under it, you should be able to check Control + Alt + Backspace. Once done, click on Apply.