Everybody can access the garden, but plots are supposed to be "private". As they're not protected, it's kind of hard to keep people away. My girlfriend faced some similar situation with her garden plants, and had quite a nice idea.
On a small slate board (a little bigger than the one used to identify the plants) she used some chalk to write (as accurately as I can translate it):
This area has some very fragile, delicate and sensitive seeds and plants. Please be very careful when walking around, and don't make any cut, as it will harm them.
If you're interested in knowing them better or even get a sample, please leave a small note and I'll get back to you. I'll be happy to share.
Guess what? People didn't touch or mess anymore. She never had a single note on the slate board. Once, a person sharing the same area asked her about the plants, and got a sample.
The only damages were made by animals. Unfortunately, these are free to go and can't read :)
Why did this work? I believe that people, when facing this kind of "warning", realize that it has happened before, and that not only you know it, but that it's harmful, which can help them understand how their actions have affected others, or can affect others in the future. So, they may have some kind of "revelation" and keep away, thinking "yeah, that's bad, I can't do that...".
Especially because you wrote that you're willing to share. Not doing that will often make people feel like if they were stealing from you. An idea that may not have even crossed their mind at first!
Some still will, but some will think and stop. You can't be sure before, because you can't read people's mind, and know how they'll react. This worked for her, hope it will for you too.