Sometimes just harping at them makes them stop as in it draws attention to what they are doing. When you see them start to stuff more food just say XXXXX stop - chew. When you start saying XXX stop as he lifts the spoon from the plate to take another bite it gradually gets him to keep the spoon on the plate - it can be full of food but it must be on the plate till he swollows. It is a learned thing and can be unlearned too.