Scrape the excess putty away with a butter knife. Work gently to avoid inadvertently scratching the marble.
Swab the affected area with a soft cloth that has been dipped in acetone or clear ammonia, rinse with lukewarm water, then wipe with a clean towel to dry.
Pour 2 tablespoons of baking soda into a small bowl. Add 1 tablespoon water and stir well. Spread a thick layer of the mixture over any remaining stains.
Cover the paste with plastic cling film and seal the edges with pieces of adhesive tape.
Remove the plastic wrap after 24 hours, exposing the materials to the open air. When the poultice is completely dry, use the edge of a plastic spatula to lift the paste from the marble. Wipe the area with a wet sponge to remove any remaining residue, dry with a clean cloth, and repeat, if necessary.