Electrical – How to turn off the power to the thermostat

So long as whatever you do de-energizes the wiring you're working on, you should be okay.

There's really just two concerns:

Shock hazard

You obviously don't want to get shocked/electrocuted, and only working on wires that are not energized is a good way to ensure this. This is less of a concern on the thermostat end, where you're only dealing with 24VAC, but there is 120V on the furnace control board.

Shorting out equipment

The other concern is when dealing with the wiring you can easily cause connections between stripped ends, which could do anything from simply turning the fan/furnace/AC on, to rapidly cycling equipment (which could damage it), to actually shorting something out -- such as the transformer itself -- and causing it to release the magic smoke, which renders it useless.

No it is not normal. There is normally a disconnect between the power supping the condenser and the condenser itself. It should be near the condenser. (the condenser is the outdoor portion of the A/C side of your system. I am certain that the disconnect you need is the box shown in the second picture. Open the front and pull out the plug. This should kill the power to the transformer. Also you could find the breaker in the house panel that supplies the furnace and secure the power there.