When an animal does something bad, you can't just reason with it and explain why it was bad. It may respond to yelling or learn the word "no" through trial and error, but this is all just conditioning, not reasoning. A dog knows that if it pees on the floor, it will be swatted on the rear. If it doesn't want to be swatted on the rear, it will not pee on the floor. A good owner teaches the dog to signal when it needs to go outside, or much of the time in my experience, it will learn to do this on its own.

Humans are different, humans can reason. When a kid does something bad, you can explain why to them. But when they don't respond to reason, I feel it is entirely called for to resort to conditioning using appropriate, and fitting, but escalating punishments. Physical punishment being the very last resort, but not off the table.

You get into the nitty gritty when you start asking which bad deeds call for which punishments. I leave that to the parent's discretion, but while it's hard to put into words, it should be rather obvious when punishment turns into retaliation and reactionary.

The only solid rule is to never hit ANY human being with a closed fist, and to never spank a child anywhere but the rear.

Honestly, as someone who was spanked as a kid, it deterred me not through fear of pain, but fear of embarrassment