I've never tried it, but people do recommend spraying the aphids with a soap solution- use the cheapest unscented soap you can find, I should think, and try it on a couple of leaves first and see if it harms them. (This is from Vegan News's site which came high up on googling: "You can make your own solution up, by adding 2 oz (56 g) of grated household soap (or soapflakes) to a gallon (4.5 litres) of hot water. Allow to cool before spraying onto infected plants. Soft soap solution can also be made from old, leftover slivers of soap. So, don't throw these away! Collect them in a jam jar, cover them with water and leave to soften for a while. Finally, stir the mixture until it becomes jelly-like and put a small amount of the soap jelly into a plant spray. You will need to dilute it with more water before using it to spray your aphids.") I'd imagine you'd have to respray if they came back, or after it rained!