Actually, if you explain it to them twice on two different occasions, I don't see why a child wouldn't understand already before reaching the third strike. They'll see how serious you are about it if you spank them a bit. They won't do it again because it hurts, and they might ask why, but the parents will say "I explained it to you three times already, and you refuse to listen. Don't hit your sister" or w/e the child did. They might even reflect on what they did and think twice the next time they're going to smack their sister, or write on the walls, etc.
It's not the same as child abuse, because you only do it to cause slight pain on rare occasions in a place on the body that can't cause long-term damage. Child abuse is leaving bruising, letting a child bleed, breaking an arm. Abuse is when you go too far - that's why it's called "abuse".
Take it this way. Casual drinking. That's not abuse; it's the same as spanking. Becoming an alcoholic is abuse, or in this case, beating your child with fists.
The dictionary says:
When you spank, you don't mean to be harmful, injurious, or offensive. Your intent is not to hurt them; your intent it to show a child, specifically children who won't listen to words, that you're the parent and you know what's best, and it's best not to fight against what you say because you have reason for it. Even if the child doesn't agree with the reasoning, they'll whine and complain about everything before the age of eight. If you let them control you from a young age, or get away with things that they shouldn't get away with, they won't understand how serious certain things are.
I said to my dad as a child "**** you". He hit me. I never did that again because I realized how serious it is to insult or offend a parent / guardian at that moment in my life. "Daddy never hit me before! Why did he hit me now? It must have been really bad what I said... I didn't realize..." If my dad hadn't done that, I'm sure I would be like my sibling of whom he didn't hit: my sister, who calls him an ******* and a bastard at least twice a day.
As said, not all children can be taught with words. Most people have friends like this, too, who don't communicate with diplomacy. For a child, you spank them on the buttocks to leave red, but not a deep red. It shouldn't last more than an hour or you've hit too hard.
S'all.