I don't know if this is relevant or not but one of the things we were told in a behaviour class focusing on "strong willed" children was to never take away a reward earned for bad behaviour. As the kids soon figure well what's the point of being good if she is just going to take it away. So the marbles would stay in the jar and the point made that no more would go in or something but what they earn is theirs. Not sure if that's of any help to you.