This is called the AI Stop Button Problem. Computerphile has a great video on this (featuring Robert Miles) which explains why this is not a reliable solution to AI getting out of control. When the AI is smarter than all of humanity combined, the only real solution is for the AI to not get out of control in the first place.

The AI would have redundancy, both in terms of its power source and also because it can literally replicate itself and have multiple instances running all over the world. Also, an army of drones that you'd have to dodge just to go anywhere near any critical infrastructure.

It's only a little bit comforting that computers still live in meatspace when you consider something like an AI-controlled Metal Gear roaming around though.

2001 Space Odyssey presents a different scenario