Exactly as you have done (up until now).
The four corner control and your predicatable presence is enough.
As far as clarifying their infiltration for passerbys, you could put up a sign that says "Leaving American Zone: Entering hippy anti-war protestor zone." Or some such.
"Leaving American Zone: Entering hippy anti-war protestor zone."
That doesn't rhyme like a Berma-Shave sign set.