It's horrible when this happens. My Aunt was a guinea pig breeder and struggled with the same problem. Two people have given you excellent ideas, though. Animals hate mothballs, so I would definitely try this, but also the eggshell idea was great. This is good for protecting your plants, and I'm sure it'll help you protect your grave as well. Basically, animals hate the feeling on their soft paw pads and it's a really easy way to go about this. I would, as people suggested, try putting something heavy on top. Perhaps find a rock that you like and place it on top. If it's appropriate and you have the space, try planting a Pennyroyal plant nearby. Cats and other animals hate the scent of these and it will keep them away. http://en.wikipedia.org/wiki/Pennyroyal I hope this all helps you. I really do feel for you. It hurts, no matter what your age and no matter how big the pet.