We have 4 Light Sussex (who I think are quite prone to it) and they all peck each other. Otherwise they seem fine, good egg production etc but bald bums!
We use the purple spray although to be honest I don't think it does anything, and we've tried the Nettex spray as well. I've given them extra protein, things to keep them occupied etc but nothing has made a difference!
I've come to the conclusion it's a bad habit they've gotten into. I don't have the space to split them apart at the moment, but interestingly we've recently introduced new chickens and a cockeral and the Light Sussex spend so long trying to curry favour with Colin they don't seem to bother with each other now!