i'm assuming you have tried the correct tool (and have turned it the right way as it's a left hand thread) so after the obvious attempts of using a vice to clamp across the flats there is little more you can do at home with out another tool.
the tool i'm talking about is basically a specific tool for this issue, it looks like a piece of metal bar with 2 flats on it with a section of smaller sized thread on one end with a large nut on the thread, you remove the nut slide the thread through the hole in the cup, the nut goes on the inside of the cup inside the frame, as you do it up tighter it grips on the cup and with some elbow grease and a big spanner will undo the cup.
try and find an old school shop who have this tool, if they don't have it don't bother as attempting to remove this with a specific tool will cause more damage to the b/b area whereas this tool i am talking about just simply does the job.
if you're near southampton GA cycles has this tool. never failed me there when i was managing that shop
oh and the hacksaw thing could work but would take ages, alot of swearing and remember you would be attempting to cut pretty hard steel