Martin, the problem with cats that can't be disciplined with noise, water guns, etc. is usually not because the human is not persistent enough, it is because the cat knows that the punishment will only occur when the human is around. If the pleasure of the behavior is more rewarding than the displeasure of the punishment, they will keep up the behavior, even if it is only when the human is not around. This is especially true when the behavior is stemming from a natural instinct, as is their instinct to cover up their waste. Your are lucky it worked for you, and it can be an effective tool, but it is not foolproof.