The Australian College of Physicians has called for smacking of children to be turned into a criminal offence. Their argument is that smacking can turn into physical abuse.
On the same grounds you can argue that we should ban sex because it can turn into rape or food because it causes obesity.
I am the father of 4 children and I know from experience that sometimes the only way to get through to a child is through physical punishment. When a two year old runs onto a road they need a consequence that is immediate and painful so they know not to do it again- trying to explain the danger of being run over by a car is impossible to a child of that age.
I am totally opposed to violence and physical abuse. We already have laws that adequately prohibit assault and protect the rights of children.
I think, however, that a simple whack to the bottom or the back of the hand communicates the message that a behaviour is not acceptable far better than any amount of reasoning.