Oh well, you will just have to play fetch every time you see a duck, LOL! Have you tried turning around and walking or running away as he is coming to you. Maybe clapping calling him or making a whistle noise to encourage him to come farther as he is coming back. It may just distract him and he might just drop it, but it is worth a try. Do you have a fenced in yard? You could try playing inside then immediately go out side and try playing. Maybe work on trading the ball for a treat and then increase distance.