Wood chisel and hammer. You will destroy the chisel, but it will get behind the nut.
Or a steel chisel and cut the bolt off.
Second this. You're going to have to go past asking to telling, and a chisel is the way to do it. But wrench is right, a steel chisel will cut it. So if you want to save the post, use a cheap wood chisel and hammer it in and then lever the nut while you crank it.