Metro style apps in Windows 8 can be closed just like traditional desktop apps. Although, the reasons for closing a Metro style app are different. When a Metro style app is no longer in use its process will automatically be suspended. While suspended CPU and memory resources are released. The only reason why I close Metro style apps is make it easier to switch between apps or if an app needs a fresh restart.
Metro style apps can be closed on a tablet by dragging your finger from the top edge down to the bottom of the screen.
On a desktop, drag the app off the screen by left clicking and holding on the very top of the screen and move the window all the way down to the bottom and then release.