How to stop or allow students/participants to share their screen or a whiteboard
While in a meeting:
1) Click the up arrow next to Screen Share and choose Advanced Sharing Options
2) Change “Who can share?” to Only Host
Get rid of gray boxes when you are screen sharing
When screen sharing, the most common solution to stop Zoom controls from showing as gray boxes to others in your meeting is to to turn off "optimize for video clip" under the More button on the Zoom controls that show while you are sharing. Only do this if you are NOT streaming a video.
As second option to get rid of the gray box from the main control bar (but not from Chat or Participants), is to enable the Hide Floating Meeting Controls option under the More button. You can make the control menu appear again by pressing the Esc key on your keyboard.
For other tips to avoid the grey boxes see the article the Get Rid of Those Gray Boxes in Zoom from TheWebiNerd.com.
How to stop or allow students/participants to write on a Whiteboard or annotate a screen share
After you start sharing your screen, the meeting controls will move into a menu that you can drag around your screen. However, to find them you may need to hover your cursor over the green meeting ID label at the top of your screen.
After you unhide the meeting controls, click on the More options button then choose “Disable Attendee Annotation”