A product called SM-90 is my personal favorite for natural pest control. It is citrus based and works well against mites, thrips, and fungus gnats (on leaves and in the soil). It also works in the soil against fungus and bacteria. Mix according to the directions and spray 80 percent under the leaves and 20 percent on top. Repeat every 3 days, up to 3 times. You can use this up to the week before harvest, but make sure to wash any residue off the plants with heavy misting.
You can also use SM-90 each time you water your plants, to kill the insect pests living in your soil. Unfortunately, if you are growing organically, it will also kill some of your beneficial micro-organisms. If you are using a hydroponic system, keeping SM-90 in your nutrient reservoir at 2-3 ml/gallon keeps fungus (and fungus gnats) at bay.