Powering off does work, at least on my unit.

If you want to verify that powering down without the USB rigmarole works on your unit, I suggest turning the light on before starting koreader. Let it do the whole screen corruption bit, then try shutting down by holding the power switch for several seconds. When it's off, the light will turn off. (I suggest turning the light on simply because it gives some feedback when the unit turns off.)

As for the [X] in the corner, I think the button is too small and too close to the edge to register the tap.