Food-grade DE will help, but since your chicken already HAS a pest, you may have to rethink your stance on pesticides. If you can rid her and the others of these things, you can use the DE to help keep them away.
Miss Prissy has an excellent method- take Sevin powder and food-grade DE and put it in the foot of a nylon or old pantyhose. Take the hen in your lap and dust the lady's bottom with the mixture, liberally, then dust under her wings. Keep checking- avoid nares, mouth and eyes of course. Treat all your birds, then use the DE in your bedding, nest boxes, on roosts, in cracks and crevices, and create a perimeter in your run. You'll have to repeat this in 10 days to 2 weeks, btw.
Also consider setting up a sand box with the DE and Sevin mixed in so the birds will dust-bathe themselves repeatedly. It can be very difficult to get rid of these things without a pesticide. Tough decision, but long-term exposure can compromise the health of your birds and make them susceptible to all kinds of other problems.