If you mean just pecking each other, that's what chickens do. Its how they keep order. Hench the term "Pecking Order". If you mean plucking feathers till their bare, pecking till they bleed, that is being overly aggressive. Which is it ?
Feather picking is hard to stop when a large part of the flock is doing it. You can try upping the protein in their feed, Feathers being protein may be one reason they are plucking. Or if you can, separate the offenders. After a week start reintroducing them one at a time. The separation can reset the pecking order and the offenders are to busy trying to rejoin the order to pick feathers. Doesn't always work as feather picking is a strong habit. Another way is get some pinless peepers. They work by blocking the birds forward vision, thus they can't easily target feathers.