Try ice spray and heat. Heat to the frame, ice spray inside (or onto) the seatpost. This will expand the frame a bit while shrinking the seatpost.
If that doesnt work, I would use caustic soda or drain cleaner to dissolve the aluminium shim. The shim is part of the frame thou. It was glued in.
Still should not be too big if a problem to get a differnt shim afterwards.