How to stop Steam Big Picture opening when you press the guide button on your XBox Controller

Scenario: You have an XBOX One Controller, or similar, and to turn off the controller you press and hold the guide button. Unfortunately, even though you have turned off guide button focuses steam in Steam's Controller Settings, the button press still opens Big Picture Mode.

Guide

• With your controller on, open Controller Settings: Steam > Settings > Controller > General Controller Settings
• Using the controller, highlight Xbox One Controller or similar, navigate to Define Layout and press A on the controller

The next step involves remapping the Guide Button to another button and then reassigning that button back to the original. This will leave the guide button unassigned so that the controller can be turned off without Steam getting involved.