Use either an insecticidal or an alternative, non-insecticidal treatment on the hair to kill the head-lice adults and nymphs. Regardless of the approach, it also is necessary to concurrently sanitize the environment (vacuuming, bagging, etc.) and nit-pick using a nit comb and one's fingers.
Beware of false advertising and misleading claims. Available products do not prevent head lice.