This lady won't ever realize that you mean her. She doesn't see it as stealing. She thinks it is okay.
Add some geese or guineas to the orchard and perhaps net the trees. That way the birds can't get to the fruit and she won't be able to pick from the street. Then lock your gates and add no trespassing signs. And perhaps some big, snarly dogs. Add a game cam and prosecute trespassers. She won't try more than once.
I think a few no trespassing signs are in order. I say keep it simple and easy to understand.
No Trespassing
No Picking
Private Property Do Not Enter
None of these will stop determined thieves. If people are known to you, and know you allow some picking, they will ask. Any determined fruit stealer will not care.
"Picking is stealing. Enjoy prison"
"No trespassing. Violators will be shot. Surviviors will be shot again." <--This hangs at my camp.
"Picky people with get their pickers chopped off." <-- Oops this one might not fly.... ETA
"Don't pick and I won't shoot."
"Go away. Mom makes her own bread."
She'll know it's for her now!
I would try to come up with nice ones but with people like that, my nice meter shuts off.
Sorry you're losing money to such selfish people.
I had posted on the easygarden about a pear tree. The tree had so many pears, branches where breaking. One day evey pear but 1 was gone. Someone said the best sign they every saw, Do not eat ( skull and crossbones) recently sprayed, the sign was up all summer. Good luck.