Disable microphone?

We have several Macs in a lab where the microphone must be disabled. Not turned down, not oops I launched XYZ and it magically turned on again, not well the output isn't stored anywhere... it has to not work, and it has to not start working. Obviously, cracking open the case with little wire cutters or a soldering iron is a little extreme :-)

With some of the older Macs or versions of OS X, someone had identified kernel extensions that drove the mic and removed them, and some of the maintenance scripts check for those kexts reappearing after updates and move them again. This is less than optimal, as it's possible an update might be applied and then the script either isn't run or bombs out, and the mic is left active. That's unacceptable.

On some of the newer Macs, I was told that one next drove both the microphone and the speakers, and they'd like to keep the speakers working. So I've been poking around a little.

On my iMac running 10.7.5, I can:

osascript -e "set volume input volume 0 --mute"

However, running a similar command or just grabbing the slider can turn it back up again. I'd like to find a way to "lock" that, or grey it out, at least for unprivileged users, but something that permanently lobotomizes it would be fine too. I was poking around in /System/Library/PreferencePanes/Sound.prefPane/ but there's nothing I can identify as the appropriate setting or file.