How do I disable WiFi hardware?

Disabling the antenna’s won’t do it. Keep in mind the higher the frequency the shorter the wave form which then translates to the needed antenna size here’s a good reference The Fundamentals of Wi-Fi Antennas. Here we are using 1/2, 1/4 or 1/8th of the wavelength for the antenna. So the exposed end of the antenna line will still be able to transmit or receive a signal (granted not very far).

So the basics question is is it possible to physically disable the WiFi? Not without altering the modem circuits them selves which is not really possible without damaging the iPhone.

Instead use How to use parental controls on your iPhone or iPad. I think that will work and you won’t harm your iPhone. Do keep in mind you’ll need to make an effort to connect to a WiFin network to download any updates.