To be honest I am frankly dissappointed in the promotion of vandalism early in this thread. What kind of action are you really promoting there? It is helping no one on either side of this situation. Let us not overreact for the situation at hand.
As Mike mentioned, this might have been an honest mistake for the guy to go through the trouble of building a stand there. If you honestly lost track of property lines and built a stand on what you thought was open land and then had it vandalized, how would you feel?
The fella needs to know he is trespassing first of all. Write a note and attach it to his stand. Maybe he was just confused and moves it. If the note is taken and nothing is done, then get the authorities involved.
I think we sometimes have a tendency to overreact when our space is infringed upon. Play it cool and resolve the issue not with revenge, but with a well thought out response.
Matt Grode