I have tried several methods including rinsing roots with new potting media and found what works best for me with plants in the house under LED grow lights.
Plants and roots are pruned back appropriately (plants to ~1' tall and roots back by 1/4 of mass.
I have Jalapeno, Golden Cayenne, Fresno and Tabasco peppers overwintering (plants in 1 gallon nursery containers). Jalapeno and Golden Cayenne are producing peppers abundantly. Plants were dug up and put in the containers with the original garden soil after spraying with neem oil twice and soaked soil with mosquito bits solution to kill any insects like aphids or soil gnats. So far so good. Watered sparsely weekly with 1/4 strength solution of MG 18-18-21 mixed in warm water.
Try the Neem Oil (a little goes a long way, follow directions for mixing, use warm water and shake well) solution sprayed on plants, left overnight and rinsed off with water then re-spray several days later and rinse again. I repeat this 3 times to ensure any aphids that did not hatch are killed. (
https://www.amazon.com/Organic-Neem-...s%2C109&sr=8-7)
Drench the soil in the container with the Mosquito Bit solution and let drain before bring plants inside. This will kill all fungus gnats and larvae. If you do get fungus gnats again just repeat the process, it does not harm the plant. I use about 1/4 cup granules to 1 gallon of water, let it sit overnight to disolve, shake and water the plants. (
https://www.amazon.com/SUMMIT-CHEMIC...s%2C105&sr=8-1)
Good luck.