Looking for a "null" Sound Input to securely disable the microphone on a MacBook Pro I want to be able to somewhat securely disable sound input via software. 

Setting the sound input volume level to 0 obviously isn't a great solution and I don't care to plug in a USB device or mic plug.

I can't find any way to system-wide disable the microphone, so I was thinking there must be a software driver that can be registered as a Sound Input Device that doesn't actually do anything.
Check out SoundFlower

https://rogueamoeba.com/freebies/soundflower/

This virtual / software sound device allows sound to be passed from one application to another (so it of course includes virtual output and input devices).

You could achieve your goal by setting the input to soundflower, but making sure nothing it outputting to soundflower... this means soundflower's input would 'hear' nothing!