I use a product called Hot Peck (there are other brands that are just as good); put it on the tail section or anywhere else that might be getting picked on. It is an unpleasant taste for the ones doing the pecking. If this doesn't deter them, especially with Rhode Island's, raise the individual separately for a stew or barbecue. Some breeds tend to be more cannibalistic and you don't want to breed that trait or tolerate it. If only one chick is getting picked on by the troublemaker then remove that one that's weak. These tend to be sweethearts and make great pets but poor flock members. In the wild, those are usually eliminated by natural selection. Similar to what I said previously, you don't want to breed these traits either, as they are undesirable for flock integration.