Take the seat off and wrap on the post with a hammer to break it loose.
If you have truly exhausted all safe methods of removal, it may be time to look at more destructive ways of removal, such as heat and or using a pipe wrench to get a better grip and buy a new post when you get it out. Just remember to lube before you insert the new one.
Last edited by Bruzer; 12-22-10 at 10:32 PM.