Something tells me I don't even need to read the other posts to know the answer to my next question. You left your card lying around didnt you?
Something you could try, which seemed to work for a mate, is to come up with the average amount you spend on them at christmas and put it on a board for them to see. Then each time they misbehave, deduct an amount in proportion to what they have done illustrating what that amount could have bought.