Help with tablet head unit otg y

[Android Central Question]

I am using a Samsung Galaxy Tab A 7.0 8GB WiFi as a head unit in my car to replace the factory stereo. This system has been great for me for 9 months. I recently decided to mount the tablet more flush in the dash and doing this blocked the volume buttons. So I purchased a PC volume control knob that plugs in via usb and changes volume with keyboard controls that I planned to hook up via otg y. I have a car charger with a micro usb charger that is powered when the key is on and the volume knob hooked to the usb plug. The knob works great with the tablet, but when I turn the key off, my tablet continues to power the volume knob and in turn, send power throughout the cars electrical system powering the door chimes and air conditioner. My goal is to turn the key on, and have a working system, then when I turn the key off have the tablet not charge or send power to anything. Is there any way to get around this? Help is greatly appreciated.