Wild ducks -- encourage or deter ??????

a pair of wild mallards have chosen my ornamental pond for their residence in the last two days or so. Normally I would encourage wildlife but I am worried about pollution of my pond which contains a lot of koi etc and my own ducks that are nearby in a pen with a pond of their own. Are my own ducks at risk of any infections or my pond/fish at risk from the obvious pollution etc. (is that poo-lution)
If I have to deter these pretty visitors I would want to do it before they get too settled. There is no nest involved as far as I can see.
What do people think.
R

yes your own ducks could be at risk from infections if these wild birds have any