Admin Can Prevent Members from sending messages in groups is a new feature introduced by GBWhatsApp. To Get this feature on your phone please update your GBWhatsApp to version 6.40. Click here to download GBWhatsApp. Do the below steps to get the feature in your GBWhtsApp. This is a server-side feature so please wait some time after installing GBWhatsApp to get this feature.
Do the below steps to get the feature.
1. Download & Install GbWhtasapp
Click Here to Download GBWhatsApp V6.40
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