Don't all creatures do this? They need to set a hierarchy. Who is the alpha male.
If someone dies in the process, so be it. The rest will live peacefully under the new reign.
Even the schooling herd fish do this. Tetras, clown loaches. cattle. goats. lions. This is nature's way.
That, or maybe temporarily isolate the aggressor in a penalty box 10 gallon tank for a week, until he learns he has no power and no territory. Might makes right.