[mikecane]

What about putting a wee piece of tape over the mic hole?

[_delirium]

Newer MacBooks don't have a separate microphone hole anymore; the mic is now located under the left speaker grille. You'd either have to tape over the whole thing (it's a pretty big grille), or open up the machine to cover/detach the mic specifically.

[incongruity]

Where exactly is that? Apple does a great job of making the mic location non-obvious.