How can you keep foxes away from your ducks?

First thing to do is check your local regulations. It may be illegal to trap or kill a fox except during certain seasons, and there may be restrictions on what can be used, and in what manner it may be used. Once you're clear on that, there are two things to keep in mind - 1) foxes are very intelligent, so trapping or tricking won't be easy, and 2) if you have one fox in your area, then you have several, and if you remove one, another will simply take its place - especially if you keep serving up delicious duck dinners. Your best solution would probably be to pen the ducks in a fox-proof enclosure during prime hunting hours, or to get a young dog you can raise with the ducks, who will protect them.