I seriously doubt there is a way to teach them not to do it. It's actually done out of love, you know? He is trying to groom you and get rid of all the 'faults' in your 'plumage'. I have age spots on my hands which drive my quaker crazy with worry and me with her chewing on them
My birds know the command 'Don't touch!' and 'No!" and are actually pretty obedient about it but the commands come AFTER they do something so, although it works in the sense that Keku Quaker stops doing it when I tell her to, she still does it all the time.