Buy a product that specifically removes paint from cinder block. Find these at your local paint store. If your paint store doesn't offer a product, ask an associate if he can recommend an online vendor.
Wear gloves and eye gear to protect your hands and eyes. Even if the paint remover says it won't harm you skin, it could cause an allergic reaction from chemicals you don't normally come in contact with. Always keep paint remover from contact with your eyes.
Spray the paint remover onto the graffiti or dip the sponge into the solution and douse the graffiti. Allow the paint remover to stay on the graffiti for 60 seconds, then scrub it away. Repeat as necessary until the graffiti is gone. Throw the sponge away and wash the cinder block with water from a hose.