I find it interesting that most people jump to the most dramatic of reasons a wild animal might be out during the day. The most likely cause is that the animal has become habituated to humans. While there is a chance the animal could have rabies or some other rare disease that causes them to behave in what you perceive as abnormal, fear-mongering and being an alarmist isn't productive or answering the OP's question, how do you get rid of a fox without killing it?
To the OP, free-ranging may not be an option for you anymore. I would suspect that it is a fox with kits, so it's looking for an easy meal. I would keep the ducks in a secure coop. Electric fencing can be an option for repelling predators. You might also think about an LGD. They can be effective at repelling foxes, although if there are multiple foxes, you may find a pair more effective.
Ultimately it comes down to what you want to do, what you're willing to do, and how much money you are willing to spend.
