How horrible. We used to have alot when we had sheep in the field at the back, not so much now. At peak times I had two fly papers( Yes I know they are disgusting, but the do work...it wasn't much fun though when I was taking one down and it dropped onto our LONG HAIRED cat...what a job we had, and she hated me after that for about two weeks poor thing.
Other than that just keep on top of them so to speak, Tansy is said to help deter them not sure if it works though, and be very vigilant in keeping everything edible covered, and wipe tops more often than usual. You should have rid of them soon, good luck