We have a fox problem on our allotment site, and asked the council for advice. They referred us to the Fox Project, who can be found online if you wish. They recommend using counter-scents to exclude the area from being a scent-marked territory, and make foxes wary of entering it. Recommended products are "Scoot", and "Get off my garden".
They say that ultrasound devices are useless, but say that a water spraying device with a movement sensor , "Watchman" I think, works well against any animal.
Unwelcome as it may be, their logic about killing foxes is sound sense: There is no such thing as an empty territory - if you remove the owner, another fox will move in