If a "Main Isolator" can presumably be inside a locked building what is wrong with a pad lock on them?
That's a thought-provoking point.
In the U.K. external neon signs have a "Fireman's Switch" (actually labeled as such) fitted on an outside wall, mounted high, with an operating dolly which can be reached with a stick to shut off the high-voltage. But there's nothing external to shut off all power -- in many cases not even an external meter or cut-out which can be pulled.
I'd go with the "Big brother is watching you and trouble-makers will be shot" approach (or something along those lines
), unless the expense the contactor idea isn't going to be too high.