Funny how the parents who don't believe in spanking are the ones who allow their children to hit them. A child isn't going to strike or hit his parents if he realizes that the parent would spank him in return.
Yeah, they need to start learning from the time they start to use those little arms that there are only two people in the house licensed to hit and that they aren't one of them.
It makes you wonder who else their children hit with impunity. With my parents, any form of physical violence - hitting, kicking, biting, pulling hair - was a capital (i.e. spanking) offense, no matter who it was directed against. Contrary to what some people think, all that comes early and easily to children. It's natural for them. They need to be trained out of it. Taught, punished if that alone doesn't work - and it's likely not to.