Here are some steps to remove heat marks off limestone:

1. Clean the surface: Use a mild soap and warm water to clean the limestone surface, and let it dry completely.

2. Use a poultice: Apply a poultice made of baking soda and water or talcum powder and water over the heat marks. Cover it with plastic wrap and let it sit for 24 hours.

3. Remove the poultice: After 24 hours, remove the poultice and rinse the surface with warm water.

4. Apply a limestone sealer: Apply a limestone sealer over the surface of the limestone. This will protect the surface from future heat marks.

5. Use limestone wax: Apply limestone wax over the surface of the limestone for added protection and shine.

Note: If the heat marks are severe, it is best to call a professional to deal with the problem.