`CurrentImage[]` will capture on image from the webcam.  It is even possible to create a live video in the notebook using

    Dynamic@CurrentImage[]

After using this function even a single time, Mathematica will not switch off or release the webcam any more, making it impossible to use the webcam from other programs on some systems.

How can I switch off / release the webcam from Mathematica, without quitting Mathematica completely?