Have you tried killing them consistently every single day with herbal paracides for a few months? Some people seem to think a few weeks or a month should do it. Have you tried zappipng for at least an hour every single day ditto?
Also, it seems to take a variety of approaches, i.e. paracides, maybe diatomaceous earth, enemas as close to daily as possible, etc.
After that, maintenance daily for life.
Also, have you checked to see where you might possibly get any reinfection? Are any people you live with possibly infected (and they always are, with something) and not treating the problem? Ditto pets need treatment.