I CANNOT STAND TO HAVE A FLY buzzing around in my house.  The only thing nastier is a grotesque fly swatter.  Haven't had one in years.  All you need is some LIQUID SOAP, any kind.  Flies, roaches, and other pests have an oily coating on their exterior which, if compromised, causes them QUICK DEATH.  Start with a spray bottle of water with about a tablespoon of dish soap, and one good spritz will wet the fly's wings and he will go down.  Another spray and he'll be dead in a few seconds.  THIS WORKS ON EVERY BUG AND SPIDER THERE IS.  Something strong like Formula 409 will even knock WASPS right off their nest, just make your solution stronger.  My mom had a large Azalea bush that was DYING because it had thousands of mites chewing on it, so we put some soap in a garden sprayer and soaped it until it looked like a shaving brush, next day all the bugs were gone and the leaves were deep green and it was good as new.  As far as I can tell, soapy water will not harm any plants.  You can even used leftover dishwater that's still soapy, and you have FREE bug killer.  The best part is that you can take part of an old paper towel or something to wipe up the dead insect, and the surface is nice and clean.  Try it and see.  I have killed bugs for years using this method and never had to wonder how harsh the chemicals in my bug spray are.