sudo srm -rf /System/Library/Extensions/AppleUSBAudio.kext
sudo srm -rf /System/Library/Extensions/IOAudioFamily.kext
sudo touch /System/Library/Extensions
This will disallow all audio input, could disable audio output, and will probably require reapplying after each system update, which may restore missing drivers.
Disable audio input hardware
The deepest level is to disable or remove the actual hardware in your computer. This can be easier than it sounds. I asked Kyle Wiens, the head of repair-guide and parts company iFixit, and he said one could melt the solder for the audio port connection on a motherboard and remove the built-in microphone altogether.
The 2015 MacBook has a removable audio subsystem.
On some models, it can be much simpler. The 2015 and 2016 MacBooks have two built-in mics, but they're close together and connected to the motherboard with a detachable cable. Following iFixit's guide to replace an audio port, you could instead disconnect the mics, disconnect the entire assembly, or completely remove the audio board.
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