Whiteflies can "suck the life out of bananas" as you put it, but it takes a VERY heavy infestation. If you are in a place where the weather gets cooler in the winter, nature will take care of the problem soon.
If they are very troublesome to you, you can mix up that 1 T soap and water spray, add 1T cooking oil per gallon too and it helps smother them. I use that suave coconut shampoo instead of soap, it smells good and doesn't seem to damage the leaves so much. Don't spray it at a sunny time, do it early in the am, late in the pm or on a cloudy day or you will burn your leaves.
Most likely your whiteflies ae coming from another plant and just hanging out on the bananas, they are not the preferred food of whiteflies, hibiscus and brugmansias are!

Just hose them off.