Prevent Apps From Running in The Background

Previously on Windows 8, when you drag a Start Screen app down to the bottom of the screen, it will close the app. However, on Windows 8.1, this action will only make the app disappear – it is still running in the background. To close the app, drag the app down to the bottom of the screen but instead of letting it go, hold on to it. Wait until the app card flips over, and only now the app is closed.

Thats how you can prevent running Apps in the Background!