I don't know who to prevent them but I do know how to have fun getting rid of them - especially if you have kids around with nothing to do.
Check this out: http://www.bugasalt.com/
Yes, I have one. And I can't think of a more fun way to kill flies!
NB El Dorado Offset, Primo Oval Junior, Traeger Model 075 Pellet Pooper, Big Chief, Kingfisher Kooker 14" gas/wood combo, PBC Brethren Edition
Home Brewer, Murderer of Squirrels, Armadillo Inspector
Flaming Pig Head Mod
Black Thermapen, red backlit Thermapen
Certified KCBS BBQ Judge