If it's on the clear side of the disc, then something can definitely be done. It's normally a PITA to take away enough substrate and get it to a fine polished state, but it can be done. Once the metal layer is damaged however, you're toast (as Tony has also mentioned at least once).

I have a MaxQ CD that someone obviously abused this way - the damage is to the top metal layer - right through the thin clearcoat. Looks like it had a little dance with some solvent. My former car changer didn't like it at all. My home player can handle it fairly well and my CDROM seemed to to a perfect job extracting the tracks.

Bruno