Once mold has penetrated the grout, the only way to get rid of it is to remove the existing grout, then replace it. All the cleaning in the world will only remove the mold from the surface of the grout and the mold will simply redevelop from the inside out. Further, cleaning the grout with harsh chemicals and cleaners such as bleach actually decay the grout over time, making the grout more porous and encouraging further mold development.