In the room, click on the i in the top right for Chat Details. Open the Options menu, then click Chat Settings (Figure 1):
Figure 1 Open Chat Settings
Select Deactivate Room at the bottom of the menu (Figure 2):
Figure 2 Deactivate Room
Confirm that you want to deactivate the room and it will be removed from your left nav.
Note: You can only deactivate rooms that you an an owner of
Note: If a room is deactivated incorrectly, orphan rooms can be accidentally created. Refer to the Knowledge Article found here on orphan rooms for further information