How to close or remove board from workspace where I am not admin in board but in Workspace?

[Vivek]

I am admin in workspace but not in some of boards, those boards where created by members who are not part of workspace now. So how to remove those board out of workspace where I am admin and those (admin of the boards) are not even members of this workspace or close those boards or change workspace of such boards?

[Michael Pryor]

"Business Class Workspace admins are able to close any board within their Workspace, even if they are not an admin of that particular board or if it is a 'private' board within the Workspace."

https://help.trello.com/article/777-closing-a-board

Alternatively, create a new workspace, move your boards into it, and abandon the old one.