I have the same problem. My lowest hen is being plucked (her neck feathers) by the next-lowest hen who is very aggressive with her. I've smeared Pick-No-More lotion on her hackle feathers... to no avail. I've sprayed Blu-Kote on them, and still she's getting plucked. She hates the smell, I think, so I give her seed treats for tolerating my daily treatment of her feathers.
I've even put the bully, a black australorp, into daytime time-out...locking her in the run for hours, while the others free-range. They all free-range most of the day, they get a high-quality organic feed, plus oyster shell and some days I give them protein treats, too. The time-out hasn't really killed her bad habits, yet.
The lowest hen is my prettiest, too. An EE, with full beard and muffs. And she's got beautiful color contrasts in her neck feathers, which makes the plucked areas EVEN more obvious. I feel sick about this. I think the bully has only been able to break the feathers, leaving about 1" - so I'm grateful there's not been blood, so far. But, what can I do? I'm afraid to leave them for even a 2-day vacation. Any advice besides culling or selling?