My 99% sure fire process is a 1/2in threaded cro-mo rod down the center of the cut of seat post, with nuts and washers threaded on to the rod thru the BB opening, then a short section of gas pipe with an ID fractionally larger than the OD of the seat post, and another gas pipe converter section threaded on, that has an ID slightly larger than the threaded rod, more washers, nuts (note all the nuts should be Grade 8 or better), LOTS of lubricant. The hardware gets SIZZLING hot. Pull up until touching the inside of the gas pipe converter, disassemble, cut off, repeat until removed.

This set up is quite cable of exerting over 10000 lbs of force.

There may be some cosmetic damage around the top of the seat tube.

What we are fighting here is aluminum corrosion growing into the surface roughness of the inside of the steel seat post. This cannot happen with a black anodized seat post - the outer layer is already corroded (anodized) and sealed. It is also less likely to happen if the inside of your seat tube is polished. Cheapest thing is to properly grease your seat post.