I was starting to think he would have to end up on my rotisserie too, and I was dreading it. I have to run the yard on my own - my sweetheart wants very little to do with my hobby, and that is fine - and I had no interest in killing a named pet in the side yard on a sunny afternoon. Been there, done that. I am glad I know HOW it is done, but I will wait until there are no local farmers raising beautiful ducks & keep baking up a storm with all my duck eggs.
I started out only wanting girl ducks. In the chaos of picking up my first 3 ducklings I ended up with 2 boys. It is hard for me to give up on an animal once I take on the responsibility of raising them, and I wound up trying to make it work. My drake charmed me into giving him a shot.
I had a sense he was after my chickens in a less than G rated way one day & sat there watching him. Then, he tried it, mounting my EE hen in the backyard.
I started to range them separately, and he became increasingly angry when he couldn't get to the chickens.
One day I came home & he had found a way through the fence & was chasing my Wyandotte. He grabbed her by the chest & started to drag her into the bushes. I got to him, and scared him off & went to lure the chickens back into the coop so I could contain the drake & fix the fence. As I escorted the girls down the hill he came FLYING down the hill - he'd never so much as gotten an inch off the ground before - and landed on top of my Wyandotte & started brutalizing her, right on top of my foot. I grabbed him & tossed him in his duck house where he proceeded to thrash about & throw himself at the hardware cloth trying to get out the entire time I wrote his Craigslist ad.
He is a good boy now, with 12 more ducks than he had before & is not causing problems at all - probably too worn out - at his new home. My girls started laying the next day, and haven't stopped. Honestly, I think he was stressing them out, too.
Good luck with your boys. Everyone told me to add more girls & I was waiting for my order of ducklings, but he was gonna kill a chicken & sometimes you have to do what is best for all, not just one.