Remove the source - they are attracted to food, tall weeds etc. Fly tape is inexpensive.
Victoria...People put a penny in a small ziplock baggie and hang on porch but you can also use the same method and drop a penny in a small clear glass or jar I understand. Personally I have never done it but have seen it done and they say it works!
When we were in MX there were medium sized plastic bags filled with water tied off and hung around the patio cover. Flies see them as a large eye and usually stay away. I did this for a few years and it worked.
I'd take a guess they think it's fish eyes!