and if possible, lock the box.

This is exactly what we did when I was in the Navy... lock it and tag it. Then someone has to be pretty dim to just throw the switch.

Actually you could probably go even further... I believe that even mains circuit breakers can be pulled out of the box... you could just do that, then there is no switch to throw. And if that is the case, I guarantee that you are isolated from the mains... you will see the gap left by the breaker.