Stitch is YOUR horse. Period. If I thought someone was riding my horse, I would be more than furious.
No one, except the BO, should even be touching him without your written permission. Period. Make it very clear to the BO that no one is to touch him and that no one (except your farrier) has your permission.
I don't mean this in a derogatory way, but you allowed this girl to share your tack locker and you have allowed her to use your tack. You've "set yourself" up for this in a way, by letting her get away with using your tack.
I wouldn't let anyone share my tack locker, but in your case, the very instant I suspected someone was using my tack locker, I would send them a text message (so you can document) that I need their stuff out of my locker in 48 hours, or their things will just be set out on the concrete and I will padlock my locker. If no reply, I'd send several more text messages to document.
Once you've got your tack locked up, I like the Kool Aid idea. Even funnier if she hops on him bareback in the pasture to be sneaky, and she's goes home with a purple butt.
In your commons area, or on Stitch's stall, I'd post a sign that no one except the BO and your farrier is to handle him (yes, that includes petting) without your written consent.
A lot of this sounds harsh, yes. But this is YOUR horse we are talking about. Some people need to learn what is NOT theirs because they have no sense of it.
I also would not confront her about it until you have some sort of video evidence that she is riding him. Although if you post signs that no one is to even touch him, you can at least call her out on that because you have caught her petting him.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.