If you're the admin of the group, follow the below steps to disable the message sending function for all group participants.
- Head to the 'Group Settings'
- Tap on 'Disable messages for all except admin' and done. You can turn it off by following the same steps.
Once you disable messages for a group, the group members will see a message - 'Group Admin has disabled message sending for other participants.'
PS: Only the group admin can share messages in the group if the 'Disable Messages' feature is turned on.