As a moderator, you can restrict all participants to activate their camera in the session.

Go to Settings in the navigation bar.

A popup window will appear showing three categories. Go to PARTICIPANTS > Can share camera.

Turn it on to allow your participants activate their cameras.

Turn it off to prevent participants activate their cameras.

This will only affect participants.
Other moderators in the session can still turn on their devices.
These settings have no effect in Rooms since all users become moderators there.