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