As a host, you can disable the chat in any session with just one click.
From within the event, click on 'options' from the top righthand corner.
Select 'Disable Chat' from the dropdown menu.
If you want to hide the chat from your view, rather than disable it for your attendees, just click the minimize icon in the top right of the chat.
To bring it back in view, select 'view chat' from the bottom of the screen.