Hi JJ,
You can spend lots of hard earned cash on fancy gadgets to terminate flies that enter your home but its fairly easy to make a home made fly trap from items you may have laying around the house.
Take a 2 litre plastic drinks bottle, cut the top off to form a funnel shape.
Drill a hole in the cap approx 1/4 inch diameter.
Put some bait in the base of the bottle to attract the flies (up to you, you don't need much).
Place the funnel cap facing down in the bottle, the put some tape around the top to hold it in place.
The flies can get in but can't get out and will die, but at least they die with a full stomach.