Spanner under the crank, so it supports it. Clamp this into a vice. Then use an appropriately sized socket / socket extension br that rests on / in the bottom bracket axle.
Heat the outside of the crank with a heat gun and proceed to twat the socket/extension out and thus the bb.
Edit: Or, cut the BB axle so only a small portion is left in the crank, and instead of the spanner, support the crank on a slightly open vice.