Imidacloprid is the only insecticide that works against them, but I understand very well all the objections about it. Neem is basically useless, unless you manage to soak the whole plant in it as it only works by contact.
If you want to try something different and environmentally harmless, try using a reflective mulch. The rationale behind it is to increase temperature and sunlight all around the plant, thus creating conditions less favorable to the spread of several pests.
Reflective mulch was originally devised to help the yield of warmth-loving crops such as eggplants and bell peppers in cooler climates but it was found to be very useful to fight pests such as aphids and White flies, bringing down their numbers and hence allowing natural predators to keep populations under control.
To see if the White flies you have are susceptible to it, buy the the largest disposable aluminum pan you can find, cut it in half and then cut a hole in the middle for your plant and cover the ground like you would with any mulch. To water and fertilize the plant merely remove one or both halves of the pan.
Given the cost and labor involved it's worth a shot.
