Can you tell me which foods to avoid on a juice cleanse/fast?

Avoid fasting. The real question is, not which foods to avoid during a fast, but whether fasting itself is a good idea. The colon does not benefit from juice cleanses and fasting. There are no toxins build up inside the colon, despite what you may read on the Internet. It cleanses itself via bowel movements. Eating a high-fiber diet with lots of liquids is the best thing you can do for colon health. Good luck.