Depends what they're on - if its non edible plants, use any insecticide that treats for them. Systemic insecticides will keep them off for up to a fortnight, contact insecticides will only kill those present when you do spray.

If its a food plant, you have to be much more careful which spray you use - some are marketed for use on edible plants and all will be contact only.

Soap & Water spray - is all you need or a powerful hose to wash them off

Greenfly are quite easy to remove and squash, whitefly are a bit small for that.