I like Dallas's suggestion of closing the hives and putting out traps for the yellowjackets. If they are drawn to the honey in your hives then they will go to sweet baited traps. Also, this may be a shot in the dark, but see if you can locate any of their nests, by following them. You could even catch one, and carefully attach a feather or light string to it to aid you in seeing where it goes, then before light, night or early morning, kill them.