My Dad wrote about cannibalism in his book on poultry production: it can be reduced by reducing crowding, feeding chickens whole oats, adding salt to their diet temporarily, or by debeaking them. Although debeaking looks bad, it actually solves the problem completely - and is routinely used in commercial poultry production. It requires removing about 1/2 of the UPPER BEAK ONLY. The chicken can still eat and function just fine (Dad did it for years and years with our chickens growing-up, and never had a problem). It will bleed, though, and so have some treatment for bleeding readily available when you do debeak them. Overall, though, know that this will prevent them from hurting each other - which they can do very readily with fully functional beaks.