Most pyrethrins should attack them as a contact spray.
But the underlying issue should take some attention as well. White flies usually group on weakened plants in the shade, especially if they're watered too often. If whitefly becomes an issue, I believe it's good to check if you can change the conditions that make them flourish.
My father in law has heaps of them on his beech hedges. He irrigates through a drip hose and he waters six times more than he should. The result is constant fungal outbreaks in ALL the plants in his yard, as well as constant white fly issues.
