The cat probably uses the dirt because it is handy, and in a nice quiet corner. It is only doing what comes natrually, unfortunately.
You say you can't seal it over. You may not be able to seal it over with cement. And plastic won't do, because the cat can tear through plastic. But have you considered plain old wood? Plywood, or planks, whatever is handy, cheap, and can fit down into the basement in one or more pieces?
You say you can't seal it over. You may not be able to seal it over with cement. And plastic won't do, because the cat can tear through plastic. But have you considered plain old wood? Plywood, or planks, whatever is handy, cheap, and can fit down into the basement in one or more pieces?