I've done it with hacksaw blade. There are handles that let you use hacksaw blades open-ended. Set up the blade in the handle so that it cuts when you pull on it. Then just lower down into seatpost tube and saw away. Will take about 30-45 minutes for the cut. Use some cutting-oil so you don't clog up the teeth of the blade with aluminium shavings.
Personally, I would just clamp it in my mill and use a wide end-mil bit to shave away the entire seatpost.