jQuery attr(‘onclick’)

Do it the jQuery way (and fix the errors):

$('#stop').click(function() {
     $('#next').click(stopMoving);
     // ^-- missing #
});  // <-- missing );

If the element already has a click handler attached via the onclick attribute, you have to remove it:

$('#next').attr('onclick', '');

Update: As @Drackir pointed out, you might also have to call $('#next').unbind('click'); in order to remove other click handlers attached via jQuery.

But this is guessing here. As always: More information => better answers.