What size tank are they in?
I'm really surprised as Pentazonas are a peaceful fish. I have 6 in our tank and they are fine together, they end up shoaling with our Ticto Barbs. You could try adding 2-3 more and see if that helps.
That's very unusual. I have eight Pentazona Barbs which I keep with Pencilfish and Cardinal Tetras. The Pentazona Barbs keep to themselves, but don't even show aggression to their own kind. Having said that, there are no rules of course; there will always be 'rogue' fish of any species. Do you mean the Barb is bullying the other Barbs, or bullying the harlequins?.