We have a bumper crop of flies this year due to moisture. The only way to effective eliminate flies is to eliminate the breeding source. You can catch and kill some flies with a sticky fly trap with pheromones; but this is a short sighted solution. For long term elimination you must find the breeding source and eliminate it. Flies need moist organic material such as manure, garbage, lawn clippings, decaying vegetables and fruits, or soils contaminated with any of these materials to lay eggs in.
Flies inside a building have in most cases entered from the outside. Therefore, preventing access to the building is the best defense. Cracks around windows and doors where flies may enter should be sealed. Well-fitted screens will also limit their access to buildings. Outdoors, regular removal (at least once a week) and disposal of organic waste, including dog feces and rotting fruit, reduces the attractiveness of the area to adult flies and limits their breeding sites. Garbage should not be allowed to accumulate and should be placed in plastic bags and held in containers with tight-fitting lids. Garbage should also be placed as far from a building entrance as is practicable. In general, poor exclusion and lack of sanitation are the major contributors to fly problems. Changing exterior, dock, and garage lights to yellow lights can also help prevent attracting flies to the structure.