I'm new here, but i just finished reading a book on parenting that was very helpful to me. It makes it clear that you have to differentiate between spanking and hitting. Children need boundaries to feel secure. If you are telling your child "Don't do that. ...DON'T do that! ...DON'T DO that! ...DID you HEAR ME? DON'T DO THAT!" and then hit the child, it is clearly abuse. In that scenario, you have waited till you are angry, and taught the child that it is OK to hit when he/she is angry. Also, you have taught him/her that you are not serious the first time.
The book advises to switch a child, which is illegal in some states. Look at your state laws and follow their guidelines. I'm pretty sure that all states will forbid hitting in anger. teach your child that there are consequences for doing wrong. He/she will thank you later. I thanked my parents! But remember, you are teaching. Children are not morally mature. They will not automatically know what they should and shouldn't do in different situations. They are also VERY selfish, and will test you. Be patient, but firm. They will be happier when they know exactly what they can and can't do.