Dielectric corrosion, your only hope is to crush the seatpost enough to break it from the seat tube wall, I think I would saw down the post, the full length of the slot in the seat tube. From there use a cold chisel(or whatever) and hammer to start folding the seatpost in on itself, hitting on each side of the slit you have made. If the post is stuck down lower, you could drill holes(say 3/16) in the seat tube 180% from each other but at different heights on the seat tube--now insert a punch and beat like mad--you might get the required deformation, in enough spots, to break the bond. The holes in the seat tube(if left to 2 or so) would be of no structural consequence or ,if you must, could be fill-welded.