On Wed, 27 Aug 2014 21:58:28 -0700 (PDT),
wrote:

simple,Green scouring pad and a little water if it takes more than that then use a 120 grit Sandpaper these are stains they don't just magically disappear I've snapped lines that were bogus and tried to cleaning them off minutes later and they were extremely hard unless you have a little water and Green scouring pad it works like 400 grit sandpaper and if its been in there for a long time you may need to go a little more aggressive with sand paper yeah that takes some elbow grease it's a stain most times when you snap lines on a slab you want them to be permanentbecause they're usually being covered up by framing

Bah. Some chalk is more permanent than others. You have to pick the
correct one for the task. It may wash off easily or last longer; like,
say by walking on a concrete foundation ready for walls. YMMV