I'm going through the same thing right now. It's most likely now a habit. I feel for you, I really do. I know how crazy it can make a person, especially a caring owner who just wants some peace. Our SILKIE (you know...the pretty, sweetest things ever to grace the earth?) have picked up the habit. We came to the conclusion that it was bordem that started it, because they all have enough space in their coop, we tried the protein addition after the fact, only to have it not do anything at all, we tried adding treats after the fact, and even anti-pick stuff (hot-pick-supposed to taste bad. It's applied to feathers by a spray-bottle). No luck. But after considering bordem as being the beginning factor, it made alot more sense. They are cooped in every day and are probably starting to get bored.
The way we're handling it: We took the only pecker away. She is now living inside the coop, but kept separate in a small dog crate with her own food and water. She didn't like it at first but is now used to it. The idea is that she will forget she ever picked up the habit and we can start all over again trying to keep her occupied. We let her loose for a few minutes yesterday and the top roo and hen gave her a taste of her own medicine-because she lost her place in the pecking order. I'd like to keep her away for a few weeks more so I can be sure that she lost her habitual fettish, and then she can find her place on the totum pole again. This is my hope. We will try to introduce better habits-like the outdoors, whenever we build the run. It does not mean that it will work, but it looks good so far.
BUT-we tried this same thing in our layer group when THEY picked up the habit, and because we didn't begin to take care of the issue until much later, they never forgot their bad habit. We kept those particular peckers away for months and as soon as we let them all get together again, feathers went flying off the poor chickens' backs and into the pecker's beaks. The only solution to this problem was to remove the peckers. Since then, we've had feathers grow back and I haven't seen any more of this behavior start up.
I hope you can make this stop. Out of all of the bad things that comes with raising chickens-this is the worst, IMO. Destruction from the outside-in.