If the seatpost is an Ali one then Caustic soda it. Takes a while though (days not hours). You'll have to keep topping the soda up.
Otherwise it can be drilled. Slowly does it though and make sure you go through the drills in size e.g start small and work your way up. Be careful to follow the angle of the seat tube though as you don't want to drill through the frame itself. When the post becomes thin start pressing a screwdriver in to try and break what's left of the post rather than drilling it. Better to be safe than sorry.