My suggestions:
No.1 Lightly and gently sand the guitar in the part where he aplied the new clearcoat, in order to remove the new clear. You can ask a Luthier to this in order to preserve the guitar and restore it to its original condition. After polishing it, it should be restored.
No2. Put a translucent sticker or tape over the signature, but dont mess with the clearcoat again.
