Unfortunately, anything that eats off the ground or doesn't wash it's hands after being in contact with the earth (ie kids) will get worms. It's not a matter of if, it's a matter of when.
You can slow it down by fastidious cleaning, but you can't 100% prevent it without putting them in battery cages. As worms can and do migrate into eggs if the infestation gets too bad, it is in the best interest of human health that you do occasionally worm them.
Keeping them organic won't matter to your customers if they crack open an egg and find a tapeworm attatched to the yolk. I know that I would never buy from that source ever again, no matter how all-natural and healthy they claim to be.
So, I worm them twice a year, throw out the eggs for a week, and go on with life content in the knowledge that I am taking care of my chickens' health needs, and the health of my buyers. It's true that free-ranging and organic food is healthier for chickens, but there's no reason to throw out modern health improvements for chickens just because some other modern practices arn't good. It's a classic case of throwing out the baby with the bath-water.