You can try Fly Terminator, which is an outdoor product that lures flies into an escape-proof jar with liquid bait that contains fly food and a sex pheromone.
A good insecticide will help eliminate the flies. Purchase the kind that can be applied to the soil without harming it. Spray area that is infected and spray outside of buildings, doors, and screens where flies could enter.
Since they are only outside, just get a fogger at the store. A fogger is a can of spray that is designed to produce a fine mist over a large area. Set off the fogger and it should kill them.
