The new Focus' dome light isn't controlled by a physical switch. Looking in the manual, there's no apparent way to disable it. There's a sneaky trick to it.
Pardon my shaky hands...
To disable the dome light:
1) Press and hold the dome light button. It can be on or off. It doesn't matter.
2) Continue to hold until the dome light quickly flashes once. That's all there is to it. It will no longer come on when unlocking or opening doors, when turning the engine off, or any other time. This also disables the individual map reading light buttons.
To re-enable the dome light:
1) Press the dome light button multiple times with a brief pause in between presses until the dome light turns on then off twice. The second press will not turn off the light.