How to stop one cat from using the wrong litter box?

Hello,

We have 2 cats and have this issue. One cat is about 2 years old
(Aster) and we've had her since a kitten. The other (Ezzie) is 7 or
8. Ezzie would be fine to have nothing to do with Aster, but Aster
likes to play and do the normal "torturing" of the older one. She
will eat Ezzie's food, and that doesn't start a fight. But Aster
seems to like to use the Ezzie's litter box, and then that's a
problem. Ezzie will either stop going, or sometimes a chase and fight
ensues. You can just TELL that she's mad. Aster goes in her own box
like 90-95% of the time. So I don't know what causes her to do this.
I think it's either laziness or just thinking it's OK? Maybe it's a
game? But she does know the difference cause she usually uses her
own.

I make sure the boxes are both clean and unobstructed. When I catch
Aster in the act I scold her and pick her up and put her in her own
box. I think she partially doesn't "get it" and partially it's a game
to her. And who knows what goes on when we're at work all day.

Any advice besides correcting her 'in the act' and making sure the
boxes are clean to STOP one cat (Aster) from using the other's box?
Ezzie had been alone for the first 6 years of her life so we don't
want to make them share a box even though many cats do. Plus, Aster
has a sensitive stomach. We've had her tested for everything and they
say she's healthy, but knock on wood, if something does ever happen we
don't want any diseases passed back and forth, or just germs in
general.

Advice appreciated very much :-)
T, Aster, & Ezzie