Whenever you introduce hens to each other, they will always have to sort out their pecking order. Sometimes though, they take it too far and one of the hens can be bullied.
Firstly, get to the root of the problem.
Is the hen plucking feathers and eating them?
If so, she could be short of calcium. Make sure there is plenty of oyster shell scattered around for her to peck at. If she's on layers pellets/mash, then there will be some calcium supplement. If that doesn't work, you can get calcium supplement for their water.
No, my hen is just picking on the little one?
Give the hens two drinking stations and two feeding stations so that the bullied hen does not have to fight for food and drink. Put feather peck spray on the bullied hen. It stinks and should stop the bullying turning into a habit.
Important - if she's bleeding, remove her immediately or they could peck her to death. Grim but it could be a sad reality. Put her away from the others until she's healed up. If you can keep them so they can still see each other, it's ideal. Perhaps put the naughty ones in the coop and let the bullied hen free range in the garden. Let her sleep away from the others too - even if you have to resort to putting her in a cardboard box in the kitchen during the night.
Why is the hen a bully?
Your hens could be bullies because they bored, give them something to do. Hanging up a cabbage, broccoli or a peck block for them to peck at can take their mind off their boredom. CDs are also useful for entertaining them. Put some extra perches in their run. Let them out free ranging as much as you can.
What if this doesn't work?
You need to remove the bully hen for a week. Whether she is in the shed, in a dog crate in the garage or in a borrowed hen house, she must be out of sight of the other hens. When you reintroduce her to the other hens, she will be bottom of the pecking order and shouldn't cause any more problems.
Even after all this, she's still a bully...
The only way to deal with this is to get a cockerel, he won't let her take top spot!