Squish the greenfly with your fingers. Takes them about 2-3wks to come back to problematic numbers, by which time the wasps, hoverfly, and ladybirds will be in full pelt and will take care of them for you.
As an aside to other posters, are a few greenfly really worth risking your health, and that of people around you, not to mention the environment, by lashing dangerous chemicals around?
Try growing lots of marigolds around your roses - hoverfly love them, and will be more likely to breed in your garden, thus increasing your anti-greenfly defences!