How do I deactivate my microphone and iSight for security reasons?
To disable iSight and other cameras, use:

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


(tested on 10.9)

I'm not aware of a way to disable audio input without affecting output. It's the same hardware so unloading/removing the kext probably won't work. One step you can take is to turn the input volume down to zero in your sound preferences, but of course anyone who's able to log in will be able to turn it back up.