Avoid flicking on the nose, especially if your working them, you want that nose in good working order yeah? Scruffing can work, for some ferrets only! Always be gentle, its not to hurt them, just to talk in their language to say your the boss, but if you think its making her worse as in shes rising to the challenge of whos boss you need to abandon that method. Do you let them out for play/exercise? If so a small empty carrier can work as a sort of jail cell, ferrets get very board and knowing biting means five minutes in the cell can discourage them. Letting them lick treats from your fingers