You could take a file to the flats where the spanner contacts, many files have a smooth side so it wont damage the frame wile you file the BB flat.Or drill a hole and use a punch to rotate it around.One that I'd do - weld a bar to the BB shell allowing you to unscrew it.
You could take a file to the flats where the spanner contacts, many files have a smooth side so it wont damage the frame wile you file the BB flat.Or drill a hole and use a punch to rotate it around.One that I'd do - weld a bar to the BB shell allowing you to unscrew it.