Feather picking is a losing battle, especially when roosters are perfectly fine with standing quietly and letting hens shave their necks and saddles. This behavior just drives me nuts. But over the years, I haven't figured out any better way to deal with it than to separate roosters from the hens, and when I let the flock out together to free range, I purposely hold back the hens I've identified as feather pickers because they pick off their feathers while outside of the run, too.
At first I just separated the roos during the day and let them free range without the hens. At night they were roosting with the hens, and then I noticed the feather shavers were working on the roosters while next to them on the roosts. I've also tried putting pinless peepers on the offending hens, but it didn't stop them pulling feathers from the roos.
Yes, I've tried Rooster Booster Pick-no-mor, and all that accomplished was to make a huge greasy mess and the hens weren't discouraged by the awful concoction in the least.
The only way to protect your roosters is to keep them away from the feather pickers. My two roosters have their own partitioned section in a coop and they have their own dedicated run. They can still see and talk to the hens, but they are only together when I can keep an eye on the depluming hens.