Nettex spray works if regularly applied, but its a difficult habit to break Heather. We don't like using Nettex because the poor victim(s) can't preen without getting a beak full of it. Wherever possible we remove the pecker (if its just one), keeping her in a separate run and coop alongside the others for several weeks. Sometimes it won't break the habbit and she is despatched. Can be caused by boredom from too small a run and not enough scratching material. A feed of dry mash can keep them occupied for hours picking up all the pieces, raher than spending a few minutes getting their fill from pellets. This has worked for us sometimes. Can also be a protein deficiency from cheaper feed -try some chopped boiled egg, sunflower hearts or maggots/ mealworms. Should be at least 15% protein in the feed for layers.