How can I stop the fan in FNAF?
I don't know how to stop that fan in the security office. All it does is suck the power. I've tried clicking, holding, pinching it.
You can't - it is part of the unstoppable "1-bar drain". There are no interactions you can do to stop it.
According to the https://freddy-fazbears-pizza.fandom.com/wiki/Power_Indicator#Five_Nights_at_Freddy.27s page on the game wiki:
There is an unstoppable 1-bar drain that drains 1% power every 9.6 seconds (this value decreases as the nights go on), seemingly caused by the fan and the light in the Office, the security cameras running, and the lights around the building.
You can consider it part of the challenge to the game. If you could stop it (and the other things), it would probably make the game much easier than originally intended.