You can always spot a case of strike early because the sheep's behaviour is a dead giveaway. They are twitchy and often turn and try to nip at themselves to relieve the itch. Sometimes they even leap about trying to get away from it.
I find also that regular changes of pasture helps to prevent scouring as they aren't going from dearth to glut, if you know what I mean, so their bums stay cleaner, which of course isn't then attracting flies.
If you do find a case, spray them with iodine or Dettol solution once you have treated them as the smell is quite good at confusing the blow flies. The maggots emit a chemical to attract more laying flies to the stricken sheep so if you can interfere with that it helps.