How can I safely get rid of large house flies?

I've lived in the same house for 5 years and never had this problem before. I get a couple of random house flies that sneak in during the summer but never have I experienced this. The flies I'm getting look like horseflies but they seem to move slower. I mean, these suckers are just huge! I keep a clean home, all my trash sources are bagged and covered and I live nowhere near any sort of livestock. I killed 3 within a 5 minute span around my living room window yesterday. Recently, we discovered a bit of mold on the floor joists underneath the house and my husband went under the crawlspace and sprayed bleach but he never saw the flies there. We did take the vents out from around the outside of the house after he sprayed so that it could air out so I'm thinking they came from there. But there's no trash or anything under the house; just dirt! I mean, WTF! These things are mutants and I want to get rid of them. Any ideas?