Soak in lots of releasing/penetrating spray over several days
Then pipe freeze spray followed by hot water ( a few times)
Disclaimer: freezing spray may crack your paintwork
Then more penetrating spray
Get a high quality tool (some of them are terrible) that fits properly Make sure it is one that will work with the flats of a large spanner (rather than a socket drive one) Reason for this follows….
Bolt the tool into the square taper (using a washer over the end of the end of tool) to stop tool jumping out of BB teeth (I do this as matter of routine now on square tapers)
Before you apply force, heat BB shell up with a blow torch
Disclaimer: this will damage your paintwork
Give it some welly with your large adjustable spanner OR better …clamp the flats of the BB tool in a well mounted vice and turn the frame. Watch out for the vice breaking!
Before/in case you were going to consider it – do not use a hammer on adjustable spanner (the adjuster worm will break)
If all of this fails (ie the teeth strip) and you really really want to save the bike, then your last option will be to weld a socket to the BB and then use a long extension bar (worked for me after LBS broke 2 vices trying to get one out)