Knowledgebase
How to disable and enable guest newsletter subscription in Magento
By default, in Magento guests can subscribe to newsletters. They can do this from the Newsletter block displayed in one of the columns on the frontend. With the default Magento theme it's displayed in the left column. If you want to, you can disable guest subscription so that only registered customers can subscribe to the newsletter.
To do this, log in to the admin panel of your Magento and go to System menu>Configuration>Newsletter button in the Customers section on the left>Subscription Options panel on the right. In the panel there's a drop-down menu labeled Allow Guest Subscription. To forbid guests to subscribe just set the menu to No and click on the Save Config button. You can enable guest subscription again at any time by setting the Allow Guest Subscription drop-down menu to Yes.