In terms of actually turning off the GamePad, AFAIK there's no known method of doing this. When I need my GamePad turned off, I walk outside, away from my console, until it disconnects, and then turn the GamePad off, leaving my console turned on.
There's an app/plugin called Padcon which turns the GamePad's screen off, but the GamePad itself stays turned on and connected to the console.