System Settings/Input Devices

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This article refers to the 4.5.1 version of KDE SC, and might not apply to previous versions.

Keyboard

Enable Ctrl + Alt + Backspace to kill X-Server

Under the Advanced tab, make sure "Configure keyboard options" is checked. 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.