Sorry, Im not sure here what it is you are trying to achieve. Newsletters would only be received by your users if you send them. Therefore if you are unsubscribing everyone, you would simply not create any.
If you are referring to notifications, you can to that from Members->Member Settings->Notifications . When in that area, first of all change the notification types to remove the email option (set it to disabled for each). Once that is done, click to reset all members to default preferences in the top right.