Yes if it's just one chicken jail may be in order.
Only one hen is picking on her, she's a Welsummer. In fact, she picks on everyone, but mostly on this Faverolle. Her back isnt picked at as much anymore, but I'm afraid the Welsummer will injure her neck. I had a problem like this but with a different hen (as in, another hen was the bully, which was our other Faverolle) and we solved her problem by locking her up in the run while the others free ranged. Of course, she wasn't too happy and started complaining, but it worked! Next time I let them out, I'm going to try this again with the Welsummer and see if it helps, if not I'll look through all these remedies and see what works.