well you can maybe put a box over it and cut a hole just enough for the cat to go in.

P.S. Its normal for a dog to do that. But he shouldn't do that very often or he'll get sick.

The easiest solution would be to put the litter box higher up where the dog wouldn't be able to reach it. If it's at all possible, keep the litterbox in the same general location, cat's don't really like their stuff being moved around. You could also get a different kind of litter box, maybe one that is more closed in or covered, or a dog proof litter box like this one http://www.pfprivy.com/main.html to help keep the dog out. If you're willing to put in a little work though, you could always train the dog to respect the litter box. Enforce a boundary around the litter box that the dog isn't allowed to cross.

So your dog like Kitty Kandy! I have 4 dogs 4 cats 2 litter boxes. I put the cat boxes where I have control over them. My dogs aren't allowed to go. If they go there I can hear them. My cats rule the house so they pretty much kick the dogs a#s.

i use a rubbermaid tote, just cut a hole on side for the cats to get in, then i turn the opening into the corner so my dog can't stuff his head in there. works great for me. although the bottoms not totally flat, i scoop everyday, it doesn't cause much problems just took a little to get used to.

move the cat litter box so your
dog can't find it?
get your dog its own bed so it stops goin into
the cats?

put the litter box where the dog cant get to it

Solid Gold makes a powder just for that...its calles S.E.P...stop eating poop

pour lava in there