Interesting. Fig trees are almost like weed here, not expensive at all. I don't know if anyone needs to buy them since they grow on their own quite a lot.
Kiwi is not cheap but not thriving, either.
To your question... I think resilience is the best thing. Grow so much they get tired of stealing. Plants require some care. So, just be happy somebody took it home to take care of it and plant the next until all homes are full with plants ;) I mean... yeah, keep it cheap for you, as well. What are the plants that grow in abundance and on their own? Prioritize on those.
I bought about 8 pots of strawberries last spring and they started reproducing so I just need to find more and more new pots or ground for them. Some of the original plants have up to 7 or eight plants grown from them. In a chain. They overrun my tiny urban space and I would be happy to share if anybody would take care of those.
Anyway, just continue planting, that's one of the most noble things one could do.
Peace!
RE: Any good idea to protect plants from theft in a public space?