Spanking children is child abuse no matter what justification a parent uses. Some religious and cultural practices are universally wrong. Americans often condemn Muslim societies for genital mutilation in young girls. Corporal punishment is no different; it is beating your kids when there is always a better way to instill discipline.
I believe corporal punishment of children is not justifiable because it is not the right of any religious organization or institution for that matter to physically harm a child. Even if the intentions are good they do not justify the means. It is a parent's responsibility to decide how they choose to discipline children.