Besides keeping the dog away from the cat's litterbox, there may be another option.
I've read that dogs like cat scat(and occasionally their own) because it's still high in protein and stuff, thus can get more nutrition from it via a second pass through the digestive tract.
So, one way to prevent them from wanting the stuff would be to switch them over to a higher protein dog food.
Can't guarentee that it'll work at this point, of course.