What Ace said. Then buy a replacement - they aren't that expensive. Just don't hit the glass.

You want to split the nut. Don't worry about the washer part of the nut - once you get it split, it should come off.

Alternatively, you can use a GOOD cut-off wheel, and cut the nut between the hex and the flange, straight thru. Like above, stay away from the glass. I'd put a bunch of blue painter's tape around the area, a couple of layers thick, in case you "bounce" it will hit that and not the glass.