Do you know for sure that it was a fox? I ask because cats can do that and love nothing better than using nice soft soil for their toilet.

place wire mesh/chicken wire just under the surface of the soil and then they cant dig out too much soil.

I've seen plastic forks stuck in pots, or dead twigs, placed around the plants, plastic is probably best as the birds are attracted to the twigs, but twigs look nicer.

How about rose or bramble prunings or sprigs of holly?