EXACTLY!
So...
>
>On what grounds does the person getting smacked over the head
>with the shovel tell the person doing the smacking that he
>SHOULDN'T. If it's "good" for the smacker, Why should he NOT
>do it?

There is no reason. Unless the smacked can convince him otherwise or his guilt for commiting an evil act forces him to make ammends for his evil ways.

>If there is no REAL good (independent of both's opinions about
>shovel smacking), then to what standard does the person
>getting smacked appeal to in order to convince the smacker to
>stop!?

he appeals to his "good" side asking why he has to be smacked over the head with the shovel..or gets a helmet. But what if the person being smacked is a lunatic who does not respond to reason? might it be a good, to commit a "bad" act, to make him see? Dodgy territory since it rests upon the "good" that is the ultimate goal being universally agreed up by both parties. An Evil person would not listen to pleas or arguments to stop hitting people because he feels good doing it. Whether it is Good (in a true sense, to be a Good person to know etc.) or not is dependant on your own morality and beliefs and values.