How to disable macbook's mic, but not speakers?
Disabling Input

To disable the input, I'm not entirely sure.  Here's what I thought of:


You can use a internal audio input device like SoundFlower, which basically routes the audio output into the audio input.
You can mute the volume (although you said you don't want to do only that)
You could take a look around the /System/Library/Extensions/ folder and in the IOAudioFamily.ktexts file for anything useful, however that can muck up OS X.


Preventing Changes

Disabling underprivileged users from changing the settings isn't actually too hard:

$ sudo chmod 700 /System.Library/PreferencePanes/Sound.prefPane


Will do the trick.  Loading the preference pane from any user (other than root) will display an error.