There are few ways to exit a Screen Sharing session at any time during a conference. The first is to return to your Online Meeting Room and click 'STOP SHARING SCREEN' that will appear in the center of your Online Meeting Room.
Each time you initiate a Screen Sharing session you will also see a message at the bottom of your screen. To exit a Screen Sharing Session you can also click the blue 'Stop Sharing' button located on the right of this message bar.
You can move this message bar around your desktop by clicking and dragging on the left side of the bar. If you click on 'Hide' you can find the screen sharing message bar again in the system tray at the bottom of your screen (it may be grouped with other browser windows that you have open).
Finally hitting 'SHARE' in the menu at the top of your screen will also end the session.