I haven't had that problem with my duckies (male or Pekin).
I have had aggressive adopted cockerels. They were angry about the change in living circumstances. I don't tolerate aggression. Period.
I put aggressive animals in time out. Yes, for reals. It has worked for me.
For example, this morning one of the recently adopted guys would not leave his room and tried to bite me when I tried to assist him out. That's a no-no. I left him in the room, door secure, barn open (so he had sunlight). I went back later and tried again. Same behavior. I went back and tried again. Same behavior. The fourth time I went in, he sat down and allowed me to pick him up. I carried him outside and gave him some food. Then I carried him to the watering spot so he could drink. At this point, he said I could leave, he was fine.
He's still new here and he's still finding his way. My preference was for him to have walked out on his own, but he wasn't ready to. Maybe tomorrow.
I have had cockerels in the past be very upset with being put in time out for biting/aggressive behavior towards others. They would be even more aggressive towards me when I came to let them out of time out. The chickens (and my duckies) are VERY social creatures. The isolation makes them mad. But it works in changing their behaviors. I don't yell, I don't curse, I don't call names, I don't use physical violence, I don't bite back, I don't hit...I explain that behavior is not acceptable and will not be tolerated. The consequence is to go into time out.
For me, it works.
Try the time out. Maybe for an hour. Then let the male duck back in. As soon as the behavior starts again, pick him up, explain the behavior will not be tolerated, and put him in time out again. You may have to do it for several days. And wear gloves!!!!
You've probably already checked to make sure there is no bullying going on by the girl to the boy. And you've probably already checked for health issues with the boy.
The other option is to rehome the duck. If you go this path, please let the new duckie owners know what you have experienced.