I see nothing wrong with "No Trespassing" or "No Poaching" or "Mitts off the fruit". People are stealing from you, so why be nice?
But that's not what you asked.
How about
"We will gladly share fruit once we have picked what we need.
But please ask first and respect the property line"
But that's not what you asked.

How about
"We will gladly share fruit once we have picked what we need.
But please ask first and respect the property line"