They make a thing called a Nut Splitter. It is pricey but basically cuts the nut. Then the bolt can fall out. I have used "OTHER TOOLS" to cut the nut in half. Cut it long ways so you can see half the bolt after you cut the nut.
Bruce has a good idea with the cut-off wheel. Get a good brand like Dremel... not the ones from the Dollar Store. Those fly apart on the first cut.
Be sure to wear safety glasses, trust me on this one.
