When my fish at home or at work start fighting, I usually smack the glass to startle them and break it up. If that doesn't work, I reach my hand into the tank and grab the aggressor and put him in the opposite end of the tank to startle him and make him forget about the fish he was attacking as well as letting him know that he is not the true boss.
As for smacking the pacu, it's not like he ripped it out of the tank and threw it on the ground and started whacking the fish. I bet he hit it hard enough to startle it.
That definitely isn't the approach I would have taken, but it really isn't as bad as beating up a dog because it couldn't stand upright as a previous poster suggested. And in reality, how many of you as children were smacked for doing something bad?