For you to do any more than keep the place clean will likely require that you obtain a pest control operator''s license. Applying pesticides and chemical controls are licensed occupations in most areas.
However, making sure the screen doors are kept closed and hanging up flypaper is another matter. Anyone can do that.
Flies are attracted to garbage for and some to water for breeding. Treat all floor drains ... — Lynn E. Krafft, ICAN/ATEX editor