Two words: combat gnomes.
Also, yeah. If stealing a tomato is saving someone from eating a McNugget, cool. Congratulations on bringing a healthful treat to a tummy in need. However, if that's a little too Buddhist for your taste, and if you have commitments to make to local businesses who have opted to use Four Seasons as a vendor, I can see why you'd be salty. In the country, they have deer and turkeys and rabbits... here, you have little old ladies or immigrants who can't read 'keep out.'
A box or an area clearly labeled 'free' would help, or simpler signs with fewer words that explain the expectation of work in exchange for vegetables. Do you provide watering cans/buckets or bins for pulled weeds? I still think most people are inherently good (will be carved on my tombstone).
I have also seen some places zip tie mesh (like onion bags) over tomatoes and other plants so that they can ripen, but can't be plucked.