You need to spray it with something like PB Blaster and let it sit overnight, then possibly do it again. PB Blaster is designed to leach into threads and loosen corrosion so that the corrosion starts dissolving, if you can't get it out that way, then you might have to cut off flush, center drill and then use an extractor tool to back it out. At this point, I would get some PB Blaster and start spraying the bolt at the base, is this a bolt that goes all the way through, or does it deadhead in the block?