To get rid of green flies safely and organically, there are two ways you can go.

First, you can use ladybugs. Ladybugs love to eat green flies. Many garden supply centers sell bags of ladybugs that you can release in your garden. They are great for eating green flies and a variety of garden pests.

Second, if there are no ladybugs available in your area, add a couple of drops of dish soap to water in a spray bottle and spray the flies. The soapy water will keep them from flying and this will lead to them dying.