We had them all over my school when I was a child and later they showed up in our yard. As it gets cooler, make sure your house is an outright fortress; they WILL attempt to get in and harbor there for the winter. It's not pleasant.
As for controlling them when they're outside? You're looking at either pesticide or natural controls. Here in PA I have a similar problem with stink bugs, but my chickens gobble them right off the walls. It's amazing what five birds can do to the population. I don't know if birds like boxelders, but you could try putting out some birdseed in the areas heavily effected. Either that or just spray them down with an insecticide.