Drill a hole about 4mm or so in the lock ring and stop before it breaks through to the other side then give it a wallop with a hammer and punch. That should break it and pretty much just fall off without damaging anything else.
This. Or whack with a dead chisel on either side.
@matteroftaste started
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Drill a hole about 4mm or so in the lock ring and stop before it breaks through to the other side then give it a wallop with a hammer and punch. That should break it and pretty much just fall off without damaging anything else.