Lock and Unlock a Slide
Lock a slide to protect it from being edited, modified, or deleted by users with whom you have shared your presentation. A locked slide can only be opened and viewed by other shared collaborators.
There are two methods to lock or unlock a slide:
-
Navigate to Slides pane on the left, then right-click on the slide you want to lock and choose Lock Slide from the menu.
or
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Select the FORMAT mode on the right, choose Slide tab, and enable or disable the Lock Slide button at the bottom of the pane.
The locked slide will be indicated with a lock 
icon on it within the Slides pane.