I have this problem too from time to time. Two suggestions: I have had good
control by using a standard fly spray (pyrethroid based) well above the
plants and allowing the spray to drift down onto the critters. A very short
burst is all that is needed and this can be repeated every couple of days or
so. The nice thing about this method is that it is simple, hasn't noticeably
affected any of my plants and kills not only the adult flies but any larvae
which happen to be wriggling on the surface of the compost. Obviously you
have to repeat the process to control the eggs which hatch in due course.
The alternative is to grow lots more Pinguicula, esp. caudata etc which seem
very partial to little black flies. If you can get them, you could also try
hanging up a few of the sticky yellow fly paper thingies. These work quite
well for whitefly too.
HTH.
Good growing,
Adrian Arnold.