There are a few cat "scarers" that you can buy, some emit ultrasonic noise, some spray water, all are activated when cats approach, though they are intended to keep cats out and I don't know if they'd work in reverse. You can actually buy something that apparently deters foxes from approaching property, it is guaranteed to work or your money back. It is rather expensive but then so is continually buying flea treatment.

http://www.primrose-london.co.uk/foxwatch-p-1722.html