Admin Can Prevent Members from sending messages in groups is a new feature introduced by WhatsApp. To Get this feature on your phone please update your WhatsApp to new beta version 2.18.202. Do the below steps to get the feature in your WhatsApp. This is a server-side feature so please wait some time after installing WhatsApp to get this feature.
Do the below steps to get the feature.
1. Download & Install Whatsapp Beta Version
Click Here to Download WhatsApp
2. Open A Group (You Should be an admin)
3. Open Group Info and Select Group Settings
4. Now Select Send Messages
5. Now Select Only admins and press OK