System Settings/Input Devices

Information

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

Keyboard

Enable Control + 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.