How do i deter my cat from doing this?

i recently got a tortoise table for a sulcata tortoise hatchling. i have sand and coco fiber in it. i just seen my cat use it as a litter box!!! is there anything i can do to stop her? i really cant cover it, its 4ft by 2 ft and there is a log in the middle higher then the top and he likes to climb on top to say hi.