Unfortunately all you can do if they're not "yours" to give away is to keep providing resources and trying to convince them, but it may take a really terrible situation to finally drive the point home.
A male duck trying to mate a chicken can seriously injure or kill it. Ducks have external penises that male chickens do not, so female chickens do not have vaginas that can accommodate a duck. A duck (especially a bigger one) can tear a chicken to pieces trying to mate it - and they will, they do not know or care that they're not the right species.
Too many male ducks will also gang-mate a female duck, seriously hurting or killing her as well. This is especially problematic if they have swimming water, as the males piling on her can drown her while mating her.
Best case scenario they listen to you and you get rid of at least 4 if not all of your male ducks.
Less great scenario, you separate all your male ducks into a space where they cannot see or interact with the female ducks and chickens. (This is important - they cannot see the other ducks because they cant reason that a fence or whatever is keeping them apart. If they can see the girls, they'll fight for them. They cannot mingle)
Worst case scenario you leave things the way they are until a female duck or chicken gets violently mangled to death and they finally listen to you.