If I see it happening, I will smack the offender behind the head, enough to knock her off her feet. I knocked one off the perch one evening when I was closing the coop and checking things out for the night. She was pecking at my runt crippled barred rock and wouldn't quit. It really got me P O'd. You cannot be there all of the time though. How much room do you have in coop, pen, and on perches? That contributes to bullying and fighting. I have one that is a cripple and she sometimes gets pecked at on the roost settling in for the night. She gets up there before the others and squats down real low and hangs her head way down and her tail droops way down too. I think she wants to make a smaller target.
Sometimes I move her to a vacant spot away from the one doing it, and sometimes I just smack the offender off the perch. I never see the behavior in the pen or in the coop in the daytime tho.