As a moderator, you can restrict participants to chat publicly.

Go to Settings in the navigation bar.

A popup window will appear showing three categories. Go to PARTICIPANTS > Can chat publicly.

Turn it on to allow participants to chat publicly during the session.

Turn it off to prevent participants from chatting publicly during the session.

This will only affect participants.
Other moderators in the session can still chat publicly during the session.
These settings have no effect in Rooms since all users become moderators there.