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I'm troubled, because it's not a hex bolt and you've showed a picture of a very slightly rounded hex NUT.
The head of the bolt is Allen and is on the left side of the bike.
You reach this with a long extension though the hole in the swinging arm.
It's the bolt that should be turned as the nut is (was?) held by it's flats in the steel channel.
This is because it's virtually impossible to get a tool onto the nut.
If I've got this completely wrong and you have indeed been turning the bolt with an Allen bit, then apologies.
In that case I think the way forward would be to remove the rear exhaust pipe and see if you can get a reciprocating saw with a metal cutting blade, underneath and make a vertical cut through the nut and the end of the bolt. It won't matter too much if you also cut the channel a bit as all three bits are replaceable.
A jigsaw might be the best tool as the builders reciprocating saws are rather long from what I've seen.
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