I haven't had my girls for quite a while now, but I did have one who could be a bit of a bully. Finally, in desperation, I penned the bully (Lavender) in the cage inside the run, so that she could still see all the other birds and kept her there for a few days, then let her out. As soon as she started again, I put her back in the smaller caged area. After a few spells in there she decided to behave. The time away from the flock seemed to change the "pecking order" and they soon all got on well together. This may not work, but it worked for me. It all depends on the offender I would think.