You also might want to buy some bugs called "Fly Predators". They're commonly used by horse stables, dairy farms and the like because flies lay their eggs in manure, and the result is that horse stables and dairy barns would be infested with flies if they didn't do anything about them.
Flies start out life as eggs that become maggots. They then enter the pupa stage before emerging as the an adult fly. The Fly Predator insect is a tiny wasp about one tenth the size of a fly that lays it's eggs in fly pupae. The fly predator eggs grow rapidly, eating the fly pupa as they grow. The fly predator eggs then emerge from the shell of the fly pupa, and when they become mature, they in turn will lay eggs in any fly pupa they find.
If you can find the source of the flies, like a nearby garbage dump or anywhere you can find rotting organic matter, then releasing some of these fly predators in the area would curb the fly population.
What are Fly Predators - Fly Control by Spalding Labs
Just Google "Fly Predators" and you'll find a lot of information about them. Alternatively, I think if you simply contact anyone who works on a dairy farm or at a horse stable, they'd have encountered problems with flies already and might have a better way of controlling the fly population.