Access to projects and work items is determine by a user’s TFS project/work item permissions. Users without permission to access a project cannot access the maps in that project. If a user has access to a project, but not to certain work items, the user can browse the maps, but will not see the details (title, etc) of the mapped work items they are not permitted to access.
There is currently no means of restricting access to a particular map outside of these options. We intend to add access settings for maps in the future, allowing you to grant read-only access or deny access to specific maps.