Here are a few methods to try for removing stains from marble:

1. Use baking soda and water: Make a paste of baking soda and water, apply it to the stain and cover it with a plastic wrap for 24 hours. After this, wipe the area clean with a damp cloth.

2. Hydrogen peroxide and ammonia: Mix equal parts of hydrogen peroxide and ammonia into a paste. Apply it to the stain, cover with plastic wrap and leave for 24 hours. Wipe clean with a damp cloth afterward.

3. Poultice: Mix equal parts of flour and hydrogen peroxide or water and spread it over the stain in a thick layer. Cover it with plastic wrap and let it sit for 24 hours before brushing it off.

It is important to avoid using acidic or abrasive cleaners on marble, as they can cause damage to the surface. If the stain persists, it might be best to seek professional help.