How to child-proof our water heater?
 

My kid brother likes to turn off the water heater. What is the easiest way to lock that so he needs to find other ways to cause problems?

Everything I have found on-line was landlords being told to have tenants sign a form and hold them responsible for manhandling it.

Charge it with 20,000v at 0 amps. One touch and he won't do it again.

I think a nice ice-cold bath ought to do the trick.

Cause => Effect.

Gas water heater?

A picture of what switch/thermostat/whatever he turns off would be helpful.

And how about some discipline? (is it permitted any more? Maybe you should give him a trophy)

I was going to suggest razor wire at first, but decided against it.

Arm the water heater with a gun.

Lock the real knob in place with some wire and add a bunch of fake knobs to play with.

Maybe the knob could be completely removed?
Pulled off like 90s GM vehicle radio or heater control knobs.

It would help if you could describe where your water heater is located. All the ones I can remember (including the one in my current house) have been in closet-like spaces with doors. So you could just replace the knob with one that locks...