My kitten uses my bed as her litter tray ?

I have had this problem with one of my kittens but mine was to young to be taken away from his mother. mine only did it at night. so I put him in the bathroom for about a week with his litter box, food and water. that helped me. the kitten was so small he could / wouldn't climb of the bed or he just never made it to the litterbox. try to put him in a closed room for a while. it was heartbreaking for me but it worked.