Q:

Disconnect active Controller from Nintendo Switch

When my friends and I play multiplayer games on the Nintendo Switch and someone hast to leave, one controller is unused. Is there an option to remove this active controller (JoyCon or Pro Controller) from the list of active controllers?
Or is it possible to turn of a controller?
I am aware of the possibility to access the Switch menu where you can disconnect all controllers and reconnect all as if you start all over. But that could change the position of each player if they don't wait for pressing R and L. And it is not as fast as an ingame option.

A:

As mentioned in the comments, a short press of the Sync button instantly turns off a Joycon or Pro controller.  There are no other shortcuts for this.
Games which behave nicely will update the number of controllers that they support, automatically turning unused ones off for you anyway.

