Even the Bible says to whip the child with the rod as a form of punishment. It is not done out of a hatred to the child, but out of love to make sure the child goes on the right track. The bible even goes on to state that sparing the rod is hatred toward the child. The bible is known for wisdom and love even if you don't believe in it. Would it tell us to abuse kids?
It is just silly that people have turned spanking a child into a sign of child abuse. Parents do not spank their kid to be harsh, or cruel, it is to show them that what they are doing will hurt. If a child runs out in the street while a car is coming what will hurt more, the car, or a light spank?
No, smacking is not okay if "justified". Hitting a child is a form of abuse, and even if the child is unruly, such is not "justified". One must first ask what is even meant by the word "justified", and how one would justify abusing a child. There is not a reason for abusing a child, ever.