I've had head lice for 3 years now???

okay my family has been struggling with head lice for 2-4 years now and nothing seems to get rid of them!!!! we're a big family there's currently 15 people living in this house so it's a never ending cycle of head lice but I'm so tired and grossed out !! nothing seems to get rid of them we've gone through several hair treatments and prescription :((( please help