Double-camp, feed your fish at night.
You probably have mallards coming to the feeder. They are less likely to show up at three in the morning, especially if they roost elsewhere.
Another idea is to feed the ducks behind your dam with corn or something that costs less than fish food. Feed the ducks a few minutes before your fish feeder goes off. The ducks will be a long way off, eating their own feed. The reason I suggest feeding them below your dam is because when they eat, they poop. Feeding below your dam, the poop goes away from your pond, rather than in it.
