Welcome to BYC! You can do a temporary trimming with nail clippers of 1/3 of the top beak to stop pecking, but in the end you will have to change their situation as feather picking is usually caused by some sort of stress. A lot of people use pinless peepers instead of trimming; basically, they are a sort of poultry blind that makes it hard to aim to make the peck. If you know they're in a stressful situation, you could always just change the living conditions and sometimes this will stop the picking.
i think if your chickens are that stressed, you need to find a solution (besides trimming their beak). chickens do not do this kind of behavior unless something is wrong. perhaps you have too many chickens in the space, or their housing is too confining.
