Plant poison ivy in the pots with the flowers, or just gather some poison ivy leaves and rub them on your pretty plants. Re-apply after a few days and after it rains. Wear rubber gloves to protect yourself if you are sensitive to this plant. Whoever winds up with the seering rash is the thief.
My next door elderly neighbor is the mother of the woman I bought my house from, 14 years ago, and she still calls it her "Paula Ann's house". Our first summer here, she helped herself to flowers in my yard (some were here when I bought the place and others I planted myself) whenever she wanted. I watched her do it while my family ate breakfast in broad daylight and acting like she had a right to be stealing.
She couldn't understand why she had a rash after I 'spiked' my gardens with it by rubbing my perennials with it. And I will never tell her. After a couple times breaking out in a scorching poison ivy rash she left my flowers alone and moved on to other neighbors' yards. Some have come to me asking what to do and I laughingly suggested the poison ivy thing. I pretended like I never tried it but felt it just had to work.She sticks to her yard now, so I can only assume some neighbors have tried the poison ivy garden cocktail.
Of course, you could put a mousetrap in the planter, instead or in addition to the poison ivy.
