fox deterrents that work?

[Lalisa]

Anyone have experience with fox deterrents that work? We have a visitor leaving "deposits" on our decking on a regular basis. I've heard that the chemical repellents need to be constantly reapplied, which sounds like a pain. The ultrasonic type sounds easier, but I've heard some doubts about whether they work. Has anyone had experience with one, or has a better idea?
Thanks!

We have one of the ultrasonic ones - it plugs into the mains and it really does work. we used to ahve real problems before and had to scour the garden everyday before we could let the children loose. Now however, unless a vital piece of garden equipment has been left within the scanned area (such as a climbing frame, paddling pool etc) or DH has forgotten to put it back after cutting the grass, the gardne is always completely clear. We have it up by the house and it seems to cover the majority of the garden (about 80' long).