We've tried a thing called a dispatcher, which we bought from Ascott. But we've decided the best way is to shoot them in the head with the airgun. Takes two, one to hold the chicken upside down by the legs - it will relax and hang in this position - and one to shoot, since even a chicken isn't dumb enough to stand still. It is absolutely instant. Once the shot is done, sever the jugular and let it bleed.
We find the best time to do the deed is first thing in the morning when the crop and gut are empty. We pluck them warm and leave them to hang for 24 hours in the garage before we clean them.
Can't you keep them for meat?