OK - what kind of worms are you seeing in the poop? Can you be certain that it wasn't just that flies came along and laid their eggs in the poop - the eggs hatched and you're seeing fly larvae?
If you're sure its internal parasites -
First,
unpasteurized apple cider vinegar in the water. The U-ACV makes the chicken's intestines a "hostile environment" for internal parasites by changing pH levels.
Next,
chopped up pumpkin seeds in the feed for at least a week.
Soap - yes... soap. Like the blue Dawn dish liquid - just a drop or two in the water. It destroys a worm's outer slime coating that protects them from being digested... and then they die (hopefully).
Garlic - either chop up a fresh clove and add it to their feed each day for a week, or you can use the powdered kind. No - it won't make your eggs taste funny.
Also, peek at this link for some more information:
http://naturalchickenkeeping.blogspot.com/2012/11/ask-bee-help-my-chickens-have-worms.html
Good luck and let us know how it goes!