Just because a punishment method is ineffective doesn't mean it needs to be outlawed. There are certainly some that do not respond well to spanking, just as there are some who do not respond well to time outs. Should we make time outs illegal if studies show that it makes some kids more rebellious? There is also an unfortunate amount of parents who say they spank when in fact they are beating, which certainly is detrimental to the children. It's a very fine line that parents have to be educated about.
I WOULD actually be in favor of requiring people to obtain education and training before becoming a parent. You need a license and proper traning to drive a car, but not to raise kids? The problem is that stupid people won't stop having sex, so you'd end up with more illegal offspring than could be dealt with. So completely impractical, but it would certainly eliminate much of the abuse problem.