Diluted washing up liquid or water with garlic is often used against blackfly and aphids. According to Monty Don, these bugs are the first wave. If you kill them now you drive away their natural predators such as lacewings and ladybirds, which may not return to your garden to feed from their favoured pests. Not only that but the dilutions mentioned above may also harm these friendly and useful insects.
While he doesn't propose leaving all the blackfly and aphids to suck the life out of your veg, he has a point and it is all down to balance.
I usually use a well worn toothbrush, or baby's one, and gently brush many of the bugs from the plant.
I'm not that dedicated where I could banish them in their entirety anyway!