Question Proper way to handle cliend/server disconnection (scene closed)

I have a multiplayer game with 2 players.
Assuming one of the players will kill the app or press the back button (change scene),
1. how can I handle the logic to cleanup the scene from all the Networkbehaviours in common and avoid breaking the game?
2. Is there any callback that should I use, I saw that there is OnClientDisconnect but, assuming the client press the back button and a new scene is loaded, then on the Host I'll have the OnClientDisconnect callback but is basically useless because the client it's already gone and I'll have a lot of errors on the client side because he changed the scene before