Well, since it is a sculpture garden after all...why not put a sculpture of
a security guard on display? <G>

Wish I could remember the name of the artist who does life-size sculptures
of people...the guard, the tourists on the park bench...you've all seen
them...they're quite life-like.

Alternatively, put your scariest, beefiest volunteers in military-style
uniforms and ask them to patrol the area and politely educate the public.