How to undo those skinny flat washers that have the angled teeth in them?

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Hi, exactly as above, I want to undo one of those annoying flat washers that has the angled teeth in. The sort that bites into the wood in one direction, making it an absolute bitch to remove. Has anyone got any ideas? For reference, it's on the bottom of a broken light fitting that I want to dismantle.

Remove the nut it's locking? Twist hard.
Remove the chewed-in washer? Dig a small flat screwdriver or awl underneath it and pop it out.