I have a 9 month(she was 13 weeks old when I got her) old Golden Retirever and 2 cats, what I did was first get a covered litter box, with the opening facing the wall with just enough room for the cats to get in. When I first got my puppy and I caught her in the act of eating out of the litterbox, I quietly snuck up to the litterbox, hit the sit of the litterbox (lid) with my hand and said "NO" she quickly pulled her head out and ran away, the next time she did it, I repeacted the "No" with the slam on the side of the box. She now assoicates that box with loud scary no. She no longer goes into the litterbox... Give it a try and let us know how it goes.