Thanks for the reply again and i can say, it's NOT a personal workspace coz i am very clear i never do such silly move.
And after few time of refresh on pages, remove permission from all user,
i try to delete the workspace again and it still show me the same message.
After awhile, i realized that the left hand side "Workspace", the workspace i delete no longer there!
Finally kick this workspace out! but i think you might need to consider this as bug?