using the fireplace as a litter box

Starlight has started using the fireplace as a litter box. EEW! I can see that the ash and such has a litter-like quality. But, it's gross. How do I teach her not to do that?

She happily continues to use her regular box upstairs, but has just recently added this as a little extra!

HELP!!!

A good, sturdy fireplace screen that she can't jump over or move.
When we had a fireplace, we actually had to have an installed glass screen put on the thing.

Maybe try getting a second litter box. From what I've read here some cats like to pee in one and poo in another. Maybe your kitty is one that likes to have 2 separate places for each thing?