If you want to remove the seatpost in a destructive manner then I have had great success with the following.
1. Drill a 1/4 inch hole through the remaining portion of the post that protrudes from the bike. This hole should be perpendicular or 90 degrees from the axis of the post itself. Drill it where there is some density (meat) in the post.
2. Soak the area with your favorite lubricant both from the inside of the seat tube and outside. I like liquid wrench.
3. Put a 1/4 inch bolt through the hole and place a mating nut on the other end. Washers help also. You may want to hit the seatpost into the seat tube with a hammer to loosen it as the nut will prevent it from dropping inside the seat tube.
4. Turn the frame up side down. Now if you have one of those auto dent puller slide mechanisms. Attach it to the bolt using steel wire, cable or other suitable means. Use the dent puller action to slowly drive the post out of the seat tube little by little.
5. If you don't have a dent puller, I have also had luck by placing the back end of a claw hammer over the bolt head and washer and whaling the face of the claw hammer with another hammer. It is safer if you use a plastic hammer to strike the claw hammer face. This way you are not using two hardened steel objects to strike eachother which can lead to flying steel chips that are like little bullets. Again the action here is impact and given enough time and patience it will come out.
Caution, wear safety glasses and make sure you have more then one bolt combo handy as I have sheared off the head of a few on real stubborn seat posts. Still I have not met a seat post that can stand up to this method. The drawback is that it is destructive so not useful if you want to salvage the seat post.
Ray Homiski
Elizabeth, NJ USA