I realize this is years later, but I just had the same problem with the Clover pink chalk and tried everyone else's suggestions to no avail. Finally, because I had just treated a cooking oil stain on a dress, I tried Dawn dishwashing liquid and it worked! I applied it undiluted directly to the fabric, let it sit for awhile (at least 10 minutes) and then rinsed it out. When it dried, I was so relieved to see the chalk marks were completely gone. I hope this helps anyone like me coming along in 2015 with this problem.
Bonnie