My criteria for when physical punishment might acceptable...
The consequences of the child action must be worse than getting spanked (way better to be spanked than hit by a car or burned by a stove)
It shall not be motivated by parents anger. Often parents hit her kids because it makes THEM feel better. That is abuse. The parent shall be totally over their anger and be in a totally calm loving mood when they conduct the punishment.
Only open bare hands are to be used by the parent. (The parent needs to feel it just as much as the child does)
Only the clothed backside or the back of the child's hands are acceptable.
Spanking should only be considered if a verbal lesson would not work. You can not explain the consequences of a child's action to them and teach them a lesson before they are fully verbal. Once a child is fully verbal it should be EXTREMELY rare.