It is possible that someone else has distorted the shape of the seat tube somehow, possibly by trying to tighten it down onto an undersized post. If this is the case and you try to use another undersized post you may further damage it by clamping and also the post will not be properly supported below the clamp area. I am uncertain what to suggest if this is the case, but neither trying to force the correct size post in, or removing material by reaming is the right approach, IMO. Perhaps consultation with a frame expert would be the prudent course.