This information is surprisingly hard to come by.
I'm growing more and more fond of the many use cases of the Raspberry Pi devices, especially now that version 3 sports a 64-bit multi-core processor. When using a Raspberry Pi as a micro server, you probably don't want to leave its built-in wireless radios active. I have come across lots of scattered and inaccurate information, which I aggressively condensed and compiled into working form here, for posterity.
If on Raspbian (usually), run:
systemctl disable wpa_supplicant systemctl disable bluetooth systemctl disable hciuart
# Disable WiFi and Bluetooth dtoverlay=pi3-disable-wifi dtoverlay=pi3-disable-bt