I'm going to shame my neighbor for stealing avocados and apples from me
How did they steal your fruits?
I have trees growing in my yard.
I think it's fair to shame your neighbor for stealing fruit from your trees. People shouldn't take what isn't theirs, and if they do it's fine to call them out for it. I think if you let your neighbor know that you don't like them stealing your fruit then they'll probably stop.
What if I start making it a project and put up signs.
If the city is fine with it, then I think it would be a good idea to let others know it's your property and your fruits. How about instead trying to make up by giving away extra fruits? Trees always give more than what you can consume.