If you don't want attendees to have chat, you can replace chat with a Q & A pod.  This allows attendees to ask questions without chat. 

 

To create a Q&A pod:

1. Click the share icon (shaped like a monitor).
2. Click More.

 

3. Click on Q & A.

 

The Q&A pod will automatically launch.  All attendees will see informing them IM is disabled (if it was turned on).