We have equipped our technicians with "safety zone packet".
It consists of plastic poles connected with plastic chain.
It's all pretty light and painted red/white stripes.
It's arranged around the working place and efficiently isolates my guys from the audience.
Works pretty well.
From the other hand, sometimes the "observers" are really nosey.
So we decided to go for "get the advantage from difficult situation" rule. Other words: can't win them? Join them.
Simple marketing issue.
Let the customer, or his personnel, see how good we are, how skillfully we deal with their trucks, how good and sophisticated tools we use.
Generally - make the show.
Believe me, it works even better than isolating ourselves.
Of course - it requires skills and patience from the technicians, but it is worth it.