It’s not good for her to be in the cat’s feces/urine place, so that’s the main reason to keep her out.
That said, for litter safety, I would switch to Feline Pine, which are pellets that are safe for rabbits. You cannot use pine shavings at all – it causes long acting respiratory damage – but the Feline Pine Pellets (Unscented only) are recommended as a cheap option that is safe. Yesterday’s News is also cross breed safe for both animals.
I think you need to try to get her to stop. suggestions here would be welcome, since all I can think of is to give her her own litter pan in there, motivate her to use it instead and put the soiled litter and poos into it from the other one so it takes on her scent. The only kink in that plan is keeping the cat out of the bunny’s litter pan.
It might need to be off limits, like up on a platform that the cat can reach easily but your bunny won’t want to climb. (Mine doesn’t climb, but some bunnies would.)