I would do two things. First, are they in the ground or in a pot? Aphids attack weak plants and sedate it, keeping it just alive enough to feed on it.
Feed the plant. Put some coffer grounds (full of minerals) and some banana peels ( high in potassium) in a blender with water and water the plant.
Then put 2 garlic cloves, a few mint leaves and/or comfrey leaves, and the tabaco from a smoked cigarette butt in a blender. Add a pinch of chilli power.
Blend it up and pour it through a coffee filter.
Dilute it 2:1 two parts water to one part mix.
Pour the filtered mix into a spray bottle and add 1 drop of dish soap.
Shake the bottle and spray all the leaves and underside of the leaves. Drown the aphids with it.
Repeat the feeding and spraying routine twice a week until they are gone.
Here's some more info on reproduction cycles