I worked in the field of child abuse - providing play therapy for abused children, working with teen parents to prevent child abuse and supervising both statewide and national child abuse hotlines - for nearly 20 years before having my own children.
I do not know of any state in the United States that bans spanking entirely - if for no other reason than it would be too hard to enforce.
Typically, in most states, spanking is considered child abuse if it is excessive - if it leaves a mark on the child.
Many believe that spanking is excessive - period! It is often used out of anger and leads to poor role modeling when trying to teach our children how to manage their emotions. The use of force does not teach our child anything valuable about their behavior and the associated consequences.
That being said, having worked in the field prior to having my own children - I understand the pain, difficulty and frustration that the parents I worked with were experiencing far greater than ever before.