A duck dropping in now and then can be part of the charm. A pond that attracts life is doing what it should. The problem comes when ducks begin using a smaller ornamental pond regularly. They stir up sediment, muddy the water, damage plants and add a surprising amount of nutrient-rich waste.
If that starts happening, the aim should be not to encourage them to stay. Do not feed ducks near the pond. Denser planting around the edges can make access less open, and delicate plants should be protected. In a larger, more natural pond they may be easier to accommodate, but in a decorative home pond they are usually best treated as occasional visitors.