The only thing I can think of is building some type of cover - even some screening with a frame around it - that you can easily put over the sandbox any time it's not being played in. It might not be too hard to make if it's done in pieces and attached together when needed.
I'm told there are some sprays that leave an odor cats don't care for - but for such a big area that might be pricey, especially if you have to do it frequently. (I don't know if they work for every cat, either. Once I tried cayenne pepper to keep one of my cats away from a certain area. All that happened is that the kitty discovered he liked cayenne pepper.)
The offender could be any cat observing the sandbox in your yard, now or in the future. The cat is thinking, "Boys? What boys? What playground? If my Creator didn't put the sandbox out here for MY use, it wouldn't be here at all!" That's the way they think, I'm sure!