Use a vacuum to carefully suck up as many whiteflies as you can. Insect or dustbuster units are ideal.
Here's a recipe for whiteflies to try:
Mix 1 tablespoon of insecticidal soap, 1 cup of 70% isopropyl alcohol and 1 quart of water. Proceed to spray at least 2 times 7 days apart. Spray heavily to the point of runoff. Do not breath the mist of this as the isopropyl alcohol is poisonous. When used with care it can be effective. Do test for alcohol sensitivity on plants as isopropyl alcohol will dissolve the natural coating on plants that have waxy leaves. Do not use on seedlings. Though not as strong and more expensive, vodka may be substituted for the isopropyl alcohol.
Whiteflies cam be an indicator of phosphorous and magnesium deficiencies. You can correct magnesium deficiencies by mixing 4 ounces of Epsom salts with 1 gallon of water. Use as a soil soak for infested plants. We won't say that this is a cure all but it can certainly benefit your plant especially tomatoes and peppers.
For the greenhouse and possibly inside: use a yellow light bulb which attracts the flies and toasts them like a bug zapper. Unlike a bug zapper beneficial insects that might be in the greenhouse are not fried by the yellow light.
Last resort: Pyrethrum Spray a botanical poison, paralyzes them on contact. The key to using this product is to directly spray the target pest. This would be the under and upper surfaces of the leaves where they congregate. Making 2 applications 3 to 5 days apart should give control.
Use rue or eucalyptus, making a very strong tea from them. Then apply this as a spray against the adults.
Light horticultural oil can be used as a spray to suffocate the flies indoors and out.
Alcohol spray: Mix 4 ounces of 70% isopropyl alcohol to 12 ounces of water and spray. Again test for plant sensitivity as alcohol can burn. Don't use on plants with waxy or hairy leaves.