A certain amount of this is just going to happen. .ducks are pretty rough with their sexual behavior. I've even had two hens that were giving each other bald necks playing too much hop on topYes! My hen had a bald spot on her neck from him pulling so hard!
I just try to pull out a drake and put him in time out if he is being extremely aggressive and not giving the girls a break. Especially if they are chasing them around making them run too much and pinning them down on dry land.