How to "permanently" turn off networking on an android device?

I'm in the process of creating a "secure device" to store and process sensitive information. Using an Android device was the best course of action for me, since I can make a quick app, plus I can use the camera, finger print, and other goodies that comes with these. All development is going well, except that I want to have that device as isolated from any network activity as possible. For now I'm just turning off WIFI, making sure Mobile Data is off, and setting airplane mode on. But, and where is my question: Is it possible to do it on a OS level? in such a way that there is no obvious way to turn it back on? I spend more than 2 hours searching around but most results are how-tos, or things unrelated to what I want to do. Just to be clear, I do not want my app to do this, but instead, run a command or do something to the device that "physically" disables connectivity on the device. I would love if this does not require a custom android build. I would accept purchase recommendations for devices that comes without connectivity at all. Thanks!