When someone deletes the only administrator from a Kanban board it would be good to give the user a warning that says something like. Are you sure you want to delete this administrator? If you do so then the board will be deleted.

Or even more direct a warning that says "Warning you are about to delete you kanban board. I just delete a board and it was not a big deal because I did not have a bunch of cards - that said I could see where this could cause a lot of frustration for some.

Jim