How can i get my kittens to stop using my plant as a litter box?

So i have this hibiscus plant. it used to live outside but now that it's getting cold outside, i had to move it inside. i put it near a window to get light, but my kittens jumped on the windowsill and started ripping the leaves and flowers and breaking branches and climbing it. i moved it away from the window. They climb it sometimes but they like to use it as a litter box. The litter box is always available to them... They'd rather use the plant than the litter box... is there a way i can get them to stop or some sort of guard i can put on the plant? Thanks :)