For technical reasons, only entries that have been added in the app via the menu option “Add anniversary” can be deleted in Birthday Hub. Anniversaries imported from the Windows Contacts app can be hidden as an alternative (see Hide entries), which has a similar effect. However, the entries are not deleted but are only deactivated and can be displayed again as required (see Restore hidden entries).
To delete an entry you have added in Birthday Hub, open the detail view of the anniversary by clicking it and then click on the trash can icon. This will irrevocably delete the entry.