It is strongly recommended that you quit your game using the in-game menus to guarantee your progress will be saved and your PC will be returned to a normal state.

If you cannot exit a game via in-game menus, you can send a forced quit command to your PC to end your GameStream session.  

From your GameStream session

• Long press Start on controller to bring up the On-screen control.
• Using the D-pad, select Quit Game

Gamestream will attempt close the game on your PC remotely. In rare cases, the game may not actually quit completely, or the PC may not be accessible for GameStream without rebooting.