In the room, click on the i in the top right for Chat Details (Figure 1):
Figure 1 Information Button
Open the Options menu, then click Chat Settings (Figure 2):
Figure 2 Chat Settings
Select Deactivate Room at the bottom of the menu (Figure 3):
Figure 3 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