Some birds (especially hens, in my experience) are just born dominant and pick on other birds. Five weeks does tend to be a hormonal stage, but it is possible that this is simply the chick's personality. How long have you tried keeping the picker in "time out."? If you haven't already done so, I would probably keep her separate for at least a week. Hopefully, that would give the other birds time to gather confidence, and once re-introduced, she would be the "new" bird that starts lower in the pecking order.
Other than that, there isn't much you can do. The behavior may get better with age, or not. As long as she isn't seriously damaging the other birds (drawing blood) or preventing them from getting anything to eat or drink, it should be okay to leave her in with the others. It would help, though, to offer more than one feeding and watering station so that the other birds can eat/drink separately from her. Offering some type of hiding spot wouldn't be a bad idea, either.
