As one of those that has had to repair and maintain those things. (AV Specialist, local college) I offer the following.

A lot of times there will be no problem with equipment that is partially disabled. But, it has to be disabled internally so that the switch position or plug will not work even if it is accidently used. Then appropriately labled! Any good AV Specialist can do that, but if they just say don't use that position it is not good enough. Lets face it budgets at universities and colleges do not allow for everything to be replaced. So, some "safe" corners have to be cut.

And that brings up a chance to mention my favourite peeve during that part of my service. I would like to shout this from a rooftop... "if a piece of equipment has a grounded plug, it must not be tampered with" Cutting off the ground on any equipment caused it to be immediately removed from service till repaired. Equipment that is designed to be used in a grounded plug can be bloody dangerous if modified. If you must absolutely positively use it, use an adapter. That way the next person will not suffer from some fool's mis-use.

And you can flame me on this one. During my years at the college we never had one single person hurt by defective equipment.