If your flies are limited to a relatively small area such as we have with our horse paddock, then you might want to take a look at
Fly Predators from Spalding Labs. These are another little bug that has a taste for the fly larve, and actually prevents the larve from maturing into adults. It doesn't work real well to control a fly population in mid season (i.e. right now, this year) but if you start them early in the year (we start in May), then they do a really good job keeping the flies under control.
Dave