I am not sure why you are eating chalk in the first place. Eating items not usually considered food is called pica.
This website is quite good.
http://www.metrokc.gov/health/tsp/pica.htm
I suggest a visit to a physician to ensure you are not suffering any medical condition which would cause a craving for this unusual substance.
If nothing comes to light then some psychological insight might be helpful to help break the habit.
http://www.istockphoto.com/file_thumbview_approve/684363/2istockphoto_684363_halloween_candy_w_spider_ring.jpg
I would figure that eating the multi colored pill type candy pictured here; would satisfy the chalk eating experience.