System Settings/Keyboard/en

In this section you can manage the settings for your keyboard.

Keyboard Settings

Enable Control + Alt + Backspace to kill X-Server

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.

More Information

• KDE official documentation
• Keyboard configuration (Arch Linux wiki)