Some people spray their plants with a dilute solution of washing up liquid and water to get rid of aphids - I haven't tried it myself though. I don't condone murder, but perhaps you could also try squishing them? Might take a while if there are loads though!

You can try dilute washing up liquid as Annie suggested or a proprietary bug killer. If there aren't many you could gently kill them between your fingers. They've probably laid eggs though so keep a look out for new flies

I resorted to using alcohol gel (stuff used to sterilise hands) to rid my indoor plant of whitefly. They kept coming back (even worse) even after using dilute washing up liquid. The alcohol gel will not work for all plants but it did no harm to mine. I would not bother with outdoor plants unless you really like the plant. ladybirds and others will get the pests in time.

Sorry, are the flys' little black ones (could be bog flies caused by over watering), greenfly or whitefly? If the latter then dilute washing up liquid (could use cheap brand as the flys will die if it is Fairy or Aldi) is the first step. If bogfly then you need to help the soil dry out.