How can I delete the Boom application output device from my system?

Open Terminal app and run the following command(s)...

• To remove BoomDevice

sudo kextunload -b com.globaldelight.driver.BoomDevice

• To remove Boom2Device

sudo kextunload -b com.globaldelight.driver.Boom2Device

This should do the trick and you even don’t need to reboot your Mac.

Execute the following commands in a (bash) terminal window.

To uninstall Boom (v1) and remove BoomDevice:

killall Boom
sudo kextunload -b com.globaldelight.driver.BoomDevice
sudo rm -rf /Applications/Boom.app /Library/Extensions/BoomDevice.kext

To uninstall Boom2 and remove Boom2Device:

killall Boom\ 2
sudo kextunload -b com.globaldelight.driver.Boom2Device
sudo rm -rf /Applications/Boom\ 2.app /Library/Extensions/Boom2Device.kext
killall com.globaldelight.Boom2Daemon

You do not need to reboot, the effect is immediate.

Further, to wipe out Boom's profile(s)/config settings (e.g. if you don't plan on reinstalling Boom in future, or want to have a clean slate if you do decide to), you can finally execute:

rm -rf ~/Music/Boom{,\ 2}

• Check for BoomDevice.kext in /System/Library/Extensions.  If present, delete it.
• Delete Boom app [if you didn't already].
• Reboot.
• Confirm that BoomDevice has been removed from System Sound Preferences.