I'm convinced that there are occasional chickens whose brains are wired differently, and no matter what you try, they will be driven to pick feathers out of anyone in their path. I have such a hen.
I recognized that Flo had this problem very early on, around four or five months. I thought since she was very near the bottom of the pecking order, and small in size, she might have a nutritional deficiency, so I gave her special feedings of canned tuna. That didn't work. I tried separating her from the flock. That didn't work. When she returned, she was still a terror.
So, as a last resort, I tried the pinless peepers. That worked, but she still picked feathers if the victim stuck herself under Flo's beak. So I put saddle/aprons on all of her victims. That worked.
So after three months, feathers having grown back, I removed the saddles and took the peepers off of Flo. In less than two days, Flo was back to her frenzied feather-picking. So I re-installed the peepers, and the problem ceased.
Flo is a very sweet little hen, and I love her very much, but she's going to have to wear the peepers for the rest of her life, I'm afraid. I may try taking them off her again in a few months, but I doubt very much she will outgrow this problem. It's just how her brain is wired.
By the way, the peepers come in all colors. Just choose anything but red!