I have found the most amazing way to kill flies: Get a white container (I used a white Frisbee), fill it to the brim with water and add a few squirts of Joy lemon dish soap. (I have found that other brands work, but not as well.) Add the soap until the water starts to have a tinge of yellow color. Set the container where the flies are, and check it the next day. Mine was full of dead flies the next morning, so I emptied and refilled it.
I have killed thousands of flies this year. I don’t know where they all come from, but this is the end of the line for them.