Whilst editing your Products Module Structure, you can choose to secure items within this Module.
Show in list - show these items in the list view and hiding the detail view behind a 401 page
Hide - hide these items in the list view and hiding the detail view behind a 404 page.
Securing Individual Items
Whilst editing a Product, head to the "Secure Zones" tab.
Here, you can add an existing Secure Zone to this item; if you have no Secure Zones to choose from, you can find out how to create one here.
Once you have added Secure Zones, make sure to click save.