Try a Neem Oil product on the plants, Einstein Oil is the best but it is a bit costly, just follow the directions... if you have deep pockets (not too common but you never know) Dutch Master Penetrator will help dissolve the oil and make it FAR more effective, but thats usually best if you have mites not flys.
If they are coming from the soil try adding Hydrogen Peroxide to your water, it should help kill fly eggs and give the roots a little boost, also helped when this happened to me.
Other then that, try fly-paper strips that you hang from the ceiling, worked really well when I had flys and they are dirt cheap and not harmfull to plants, people or animals (if you get the non-poisoned kind, check the packaging). That will get rid of the adult ones, the other tratments just stop them from hatching etc.