I think there's a clear subjective difference between consciously spanking a child as a form of discipline, as a rational decision and with the absolute minimum force required for it to work, and hitting a child as a form of punishment guided by negative emotions. In the first case, you're doing it to improve the child's life in the long run. In the second, you're just being violent towards someone weaker than you because you're the lowest of the low.

It's like the difference between a tasteful nude photograph meant to highlight the beauty of the human body and porn, or between using dissonance in music to create tension and using it because you're a bad composer. There isn't a set of general, objective rules to distinguish the two, but if you see it, you immediately know which one it is.