You will have to find the underlying reason for this behavior, a happy healthy chicken will not pick. Something is stressing them...could be parasites, nutritional deficiency, overcrowding, too much heat or light, boredom etc. They don't do well on kitchen scraps alone and will need a well balanced protein mash(not pellets).
If you wish to see almost immediate results and if it's possible, extend a fence around part of the coup and let the chickens out in this area to forage, eat greens, 'sun/dirt bathe'
etc...allow them a little free range. If this is not possible, get a bag of decaying matter from you compost heap or an old bale and put it in the coup for them. If the behavior continues you will have to 'pick' out the culprits ;-) .
It would seem that the coup has become a 'hostile environment' for them or you have a cannibal, please don't go into the coop alone Sky