Where are you? Someone might have a big set of Stillons you can borrow. If you are in Sheffield I have some.
Otherwise, depends what tools you have and how medeival you want to get.
Hacksaw towards frame and cut 2 sides off to make flats. Put in a vice or use a big adjustable.
Through drill and screwdriver a good idea
Last resort, hacksaw blade through bb and cut as deep as you dare in 3 places then collapse the shell inwards with mole grips or screwdriver and hammer.
If you don’t have stillsons, no real neat solution and all risk damaging the frame so take it easy!