My puppy (6 months) wont stop biting my duck on the neck. How do we stop him?

he is 3/4 Golden retriever and 1/4 German shepherd. When we see him we tell him NO very clearly and he knows what that means cause he will stop and cower down or go hide, and he knows he is in trouble but he will bite the duck any time we are not around. Are other dog which is a white lab is a few years old and doesn't bother the duck. We need help, we don't want our duck to get hurt to badly. We have tried spraying vinegar on him when he does it and he doesn't like it and we spray vinegar on the duck also so he will stay away. it helps sometimes but overall he will keep going for the duck. he doesn't mind very well. any ideas? please help