You wouldn't even need to go that far.  Just lock them in a room with a slot in the door that only gave them 2,000 calories per day, and watch the weight drop off them.    I'm sure even that wouldn't get through their thick heads though.  Once released, they would return to their old eating habits (or worse, after all, they deserve some treats for all this weight loss that happened), get fat, and now they have proved that weight loss is never permanent!  Damn these cursed genetics!