From the Participants panel, the host can right-click on a particular user and select Turn Off Video to turn off that user's video stream.
The participant can turn their video back on when ready.
If the main video window is locked on a particular user, see Select Who You Want to Focus During a Cisco Webex Event.
Turn off Participants Video in Webex Meetings and Webex Events
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The hosts can now turn off a participant's video to avoid a blank screen when a participant exits a room during a meeting or event.