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.