Hiya,
Foxes hate dog's and humans and an age old humane trick is to put some dog or human hair on the ground where you don't want them to be. (they hate the smell)
If you don't have a dog, explain and ask a friend or neighbour to donate some after grooming their dog and pop it around the entrance to the hole (shove a couple of stones on top to keep it from blowing away). This shouldn't bother your cats.
Although I think foxes are lovely, I don't like them anywhere near my garden as I have 2 cats and I know it's very rare for foxes to attack cats but I always worry when the vixen in the woods is calling at night. (luckily I have a dog now so they no longer come into the garden (they used to).
I would say try the ultrasonic devices you can get, but it will affect your cats, so probably not the solution. Also, If you spot any dropings, clear these up and rinse the area well, as this will lessen their territorial bond with your garden.
With a repellent scent around the hole, the fox should move out within a couple of days taking with her any cubs, then you can block it up.
Good Luck :-)