Have you tried drilling them out? Starting with a small size drill bit. Then finishing them off with a size thats slightly larger than the bolts thread? That way you can "break" the surface contact between the nut and bolt.
So I told my "handy friend" as he was trying to drill these out "what if attack from the bottom?" and he for some reason didn't think they were bolted to the bottom. For some reason I believed him when logically that does not make sense lol.Grind the heads off with a flap disc and forget about them?
odds are high that those are stainless and the threads are probably galled like crazy.
unless you need to reuse the holes I would give up and grind them off, but they can be drilled out with patience and a sharp bit. We do it all the time at work.
on closer look, is that a wood floor? They must have nuts on the other side? Can you cut the nut or tighten them until they break?