EDIT
Since this answer has been originally posted the browser API has changed.
.stop() is no longer available on the stream that gets passed to the callback.
The developer will have to access the tracks that make up the stream (audio or video) and stop each of them individually.
More info here: https://developers.google.com/web/updates/2015/07/mediastream-deprecations?hl=en#stop-ended-and-active
Example (from the link above):
stream.getTracks().forEach(function(track) {
track.stop();
});Browser support may differ.
Original answer
navigator.getUserMedia provides you with a stream in the success callback, you can call .stop() on that stream to stop the recording (at least in Chrome, seems FF doesn’t like it)