I've tried "Rooster Booster" which is commercially available, but haven't found it to be an effective deterrent - in fact, I think some of my chickens like it. Pine tar works better but is super messy and may be difficult to find, used to be available in nurseries for use as tree-pruning sealer, may be able to find it there. Don't use Vicks Vapo-Rub (which I've seen recommended) on open wounds/damaged tissue; it is absorbed and can be toxic. Sometimes you have to remove the victim until healing up, or identify the perpetrator and remove that individual if it's only one. This is one of the most frustrating problems with chickens, isn't it? Good luck.