To my knowledge the only way to get the scratches out is buffing with cerium oxide. Also the scratches cannot be deep. I have heard it say that if you can catch it with your finger nail then it is too deep to buff out of it will warp the glass and look like the mirrors at the carnival.
I bought my nanocube used and it has some deep scratches on the front of the glass. I was worried they would be noticeable, but when its full of water I barely notice them. I wouldn't mess with it unless you really know what you're doing.