As an admin user you can make all future snag list items private by default. To do so, click on the 'Configure Snag List Settings' in the bottom right of the Snag List log page. From this page select the 'Snag List Items Private by Default' checkbox. See Configure Advanced Settings: Snag List.
This will make all future snag list items private, but will NOT retroactively mark existing snag list items as private. If you want your snag list items to be private, we recommend you do this at the beginning of a project.
Private snag list items can only be seen by 'Admin' level users on the Snag List tool and those included on the item.