- While others are screen sharing, you can click the X in the bottom left of their window to stop their screen sharing. 
 
Alternate way to remove someone's screen
- From the interactive touch display, tap the Collage button. 

- This button is floating, it can be moved around the screen
 
 - Tap View the Device List > 
 - A window will appear with the list of connected devices
 - Tap the power button by the device you want to remove