I am literally just in the process of this exact same problem. I bought an eBay bargain DMR frame, . Only the kid forgot to mention the severely stuck seatpost.
I tried all the brute force approach techniques, I also tried using ammonia left for several days and then more brute force and all I managed to do was break the bond off the clamp into the post…
What you need is caustic soda! Ghetto chemistry of the best kind!
You need to wear lots of safety gear, plug any frame holes so the solution doesn’t just pour away, and then start letting it do its thing. It’s taken me 3 bottles of caustic soda and a few evenings sitting there feeding the frame with it and avoiding the caustic volcano that erupts from the seat tube but it’s finally worked, eating the whole entire post right back to the steel…
Check out this blokes video for inspiration.
[video]http://m.youtube.com/#/watch?v=-dYnSVVDxrE&desktop_uri=%2Fwatch%3Fv%3D-dYnSVVDxrE[/video]
One thing I’d add is, don’t put a lid on the bottle you mix the caustic soda in as the gases can build up fast and be explosive… I used a Pyrex jar. It gets HOT quickly…