How do you get a dog to quit eating cat food?



My wife and I were wondering why hes barely eating his food, yet is taking massive, uh, "poops". Then this morning we noticed that were re-filling the cat food bowl MUCH more frequently than usual. My wife suggested just feeding him cat food, but it seems that dog food is formulated specifically for dogs, and vise-versa. Serious answers only, please. Answers such as "tape his snout shut", "a.30-06 from about 20 feet", or "fill his mouth with expanding foam insulation" wont help us much. Thanks.