What are ducks afraid of?
Predators of the adult duck can be large fish, alligators, crocodiles, and predatory birds such as the Peregrine Falcon, and @DavidCarlisle. The best thing you can do to get rid of ducks is to block access to any food or water. If this is not possible you have to do your best to scare them away using sounds, decoys and other repellents.