then either keep trying or deal with them, killing somthing just because it doesn't behave the way YOU want it to, it not an answer. if you run from them, then they just believe that you are a more timid "drake", next time try grabbing one by his neck, not BY his neck, but the feathers, give him a good GENTAL but firm shaking, as if you were a dominate drake, wait until he completely surrenders, then let go, if he chases you, chase him back, grab his neck feathers, and sahke him, do this until HE realizes that he's not in charge, interrupt his breeding by pushing him off the hens if you can. those are the sorts of things ducks do.
my african gander has gotten too big for his neck lately now that it's spring, he's gone after me, would I kill him if he got ahold of me, no I'd like to, but I'd just give him a good thrashing.