Text Channel Permissions
Usually, if members are still able to send messages in a text channel after being muted, it is likely because of the@everyone channel permissions being set incorrectly.
Set the channel permissions
Go to your channel settings, and navigate to the Permissions tab.
Find the 
@everyone or member role and ensure that the Send Messages permission is set to Neutral (/).
Check server wide role permissions
Go to your server settings, and navigate to the Roles tab.
Find the 
@everyone or member role and ensure that the Send Messages permission isn’t enabled.
Lockdown Command
If you have a custom member role instead of the default@everyone role, make sure that the lockdown command is set to restrict that role instead of the @everyone role.
