Give them another soak and use Poultry Protector iin the soak. Hold them under (except for their head, of coourse) for about 10 minutes. When they are dried, dust them with Diatamacious Earth, especially around their vents, but also on the back of their neck and under wings. Repeat the next week, then dust DE every couuple weeks.
This is the most natural way to do it.
Other people use SEVIN dust, which is a pesticide, which I don't partiicularly like, but I have also used it once or twice on birds that have had more miites than others. Once again,the vent is the most important spot to treat, then back of neck, under chin and wings.