I'm surprised I haven't seen it mentioned; a shock collar. No, they are not cruel. It's a training tool. You'll have to invest some time and money, but within a few attempts, that pup is going to assume that door hurts like hell!! He won't go near it. My dog gets his collar on and immediately lays down on his bed. (His safe zone where he knows he can't get into trouble and get shocked.) It's important the dog doesn't associate you with the correction or else he won't behave when you are not around. It's even better if he doesn't associate the collar and to that end, several manufacturers make fake collars for the dog to wear when not being trained or when the live collar needs to be on.