Very often, the difficult decision that a parent faces is not "Do I run over there and physically stop my child, or do I try to discourage this behaviour verbally?" but rather "Do I run over there and stop my child, or do I quickly but quietly grab the camera?"
Usually, it comes down to a question of sequence. First, photograph the behaviour, because it is hilarious. Then put a swift and stern stop to it and tell the kid to never, ever do it again.
I love being a dad.