Turn off a building. ie , On-off switch.

It would be nice to be able to remove people from a building or factory etc.
That is, and leave the building unoccupied when not needed.
Instead of just bulldozing it to free up workers.

You can pause/unpause with pause tool or with upper left corner button in building info window.

You can already do that. Select the building, at the top left is a pause button. When a building is paused it doesn't use workers (they are freed up for other buildings) and it doesn't consume power or maintenance. I believe pause tool can do this for mass amount at once.