Most microphones are visible as a small hole in the computer case. You can drive (slowly) a strong pin through it until it ceases to function (if the microphone is soldered directly to the motherboard, then there is some risk of damaging other components; this is safer if the microphone is, like mine, located over the screen, just beside the webcam). Alternatively, you can probably disable it by pouring some epoxy glue through that hole. The bright side of it is that you can test whether the microphone has indeed been killed by using any recording software.
Unfortunately, some hardware designs will put the microphone close to the hole (or small grid) but not immediately behind it. If your laptop is of that kind, then you will have to open the case to locate the microphone and let it feel your wrath. If the microphone is linked to the motherboard through a pair of wires, then cutting the wires will suffice to make it inert. (Of course, opening the case may void the warranty.)
(Alternative is to use the laptop only within a Cone Of Silence, which is impractical.)