This is going to sound crazy. But then we are talking about Derp, right? Anyway, I've read of a fellow that claims some chickens get picked on because they get more attention than the others. The solution being, to give lots of attention to the ones doing the picking. One at a time, of course.
I doubt you can do that with the whole flock but it's something to think on. That likely can't be helped, however I just thought it was something to consider.
I think that galanie is on to something here... I recently had to isolate one of my hens due to bumblefoot, while in isolation she got used to being picked up and cuddled. Initially after reintroducing her to the other girls, she would always run to me and follow me when I was in out there with them, I would pick her up and hold her and the others continued to pick on her. I did try picking the others up too but they are not cuddlers and didn't appreciate it all that much. So, I've stopped picking her up and the others as well, they do seem to pick on her less.