Unfortunately, children start testing limits before they are able to understand an explanation of why some things are unacceptable. As long as the swat on the butt* is intended to get their attention, not to actually hurt them, it can be effective. When a two-year-old decides to bite mommy hard enought to draw blood because mommy said "no", that two-year-old gets a swat on the butt and immediately put in her crib until she cools off.
* Through a diaper they can barely feel it anyway.