Both horn flies and face flies will be worse with horses that are around cattle. Control of these external parasites begins with good waste disposal and regular grooming. Sanitation means regularly cleaning stalls and lots and either composting the manure, spreading it, or shipping it away. The heat generated by composting manure will kill the parasites. Mosquitos and other pests breed in standing water, so the drier the area, the better.
Some mechanical devises work well to control pests.
Screens are effective but not practical most of the time. Bug zappers work fairly well if placed near the entrance of the barn, but can't keep up with an active breeding area. Some halter fly strips and nets are available for pastured horses and these simply dangle over the face to move the flies off. Chemical control of external parasites should be used only as a last resort. Any product that is put on the horse will discourage the insect from landing.