I'd assume the previous owner was a clydesdale. As thick as that is, I would drill and tap it to accept a slide hammer fashioned from all-thread and a hefty piece of metal. But I have access to large taps and drills and I understand someone may not want to make that investment. If it is mushroomed up enough you aren't worried it will fall down inside the seat tube, I'd try to break the bond with a good rap from a hammer.
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke