Praise works better than punishment, and it works on adults too. The trick is being specific about what the person actually did and giving the behavior a name they can repeat. Say out loud that a camper used good manners and within a day the whole table is saying please. There is one caution. A small number of people dislike being singled out in front of a group, so watch for that.
This is the short version. The full lesson, with the story behind it, is in the book.
Take The Summer Camp MBA with you, or get the field notes by email while you work through the lessons here.