As they keep coming back I would imagine that you are using a contact insecticide. Perhaps a systemic one will do the trick?

Neem oil might be worth a try. It is one of the few "organic" remedies with a residual action. Just don't apply when the temps are going to be above 29º C within the next 24 hours.

I've heard that neem oil can damage the leaves.

Normally only when the weather is too warm, and on certain plants very sensitive to oils, such as ferns and begonias.