I managed to get this working in Firefox by setting event.shiftKey to false, then calling preventDefault(), and stopPropagation(), then returning false. I then set the document model's onclick event and ondblclick events to the same function you wound up using for yourself, plus the added setting of the shiftkey to false. I also had to do this for mouseup events, because clicking and dragging was also causing context menus to pop up.
I'm not sure it can be fully 100% disabled, but this looks to be about the closest you can get it.