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I'd paint over them with BLACK or gray paint first, because those pigments tend to be the most opaque and will be best at concealing the letters. They'll still stand proud a bit, but the only other way to really hide them would be to glue (using the proper glue and material to match your dink) a patch over them, and put the new registration on that.
Solvents, sanding...might work with care and depending on what kind of paint they used. But why risk any damage to the material at all?