I dont think anything is theft proof but maybe you could make it hard enough not to be worth the effort. Most parts of the farmbot would not resell for two much as individual items and the ones that would sell would be tough to move (I guess).
So I am thinking as long as you can make it a hard to get item that would not sell for much it might be safe. To that end I think maybe you could have the farmbot lock in around the railing so it would be very hard to de-install it and maybe instead of screws put riots in as many places as possible so they would literary have to rear it up to get anything. This would kill their efforts and motivations (as long as they are not just vandals) Other thought would be to put it into a plastic cover green house (or put it on the roof) so no one could scope out the area to well, start a campaign while giving you classes or speeches that down place the cost of the whole thing so that word would not spread that it was work much (drop farmbot off the name so it cant be googled). Make a web cam and spread the word about it under the guise that anyone can look into the garden at any time but drop the hint that everything is captured and kept online.
All in all a farmbot it worth a lot as a total but I dont think anything they could and would target would be worth more then &50.