Disabling sound output devices (e.g. Internal Speakers) in OS X

One partial solution is to use the Audio/MIDI setup utility to configure a "multi-output device" that only includes the audio devices you want to use, and setting that as your system audio device. However, using this method has the major disadvantage of not allowing any easy means of adjusting the volume on the underlying devices, even if there's only one device set up.

Another issue is that if any of the devices ever get temporarily disconnected or take a little while to appear at system startup or the like, you'll have to add the device back into the multi-output device.

Unfortunately, macOS doesn't appear to have any built-in means of disabling an audio interface entirely or setting the device priority, which is supremely annoying.

I don't know of any way to disable internal speakers. However, you might look into a solution like ControlPlane, which can automatically set your default audio output (and many other system settings) based on environmental factors of your choosing, e.g. having a specific display connected, or the presence of a specific WiFi access point.

kextunload is a formal interface for unloading kexts in the Darwin OS and in Mac OS X.

Unload the system audio driver:

sudo kextunload /System/Library/Extensions/AppleHDA.kext

Load it again:

sudo kextload /System/Library/Extensions/AppleHDA.kext

If not "AppleHDA.kext", another kernel extension in /System/Library/Extensions/ may be what you're looking for.