Who is they? If you haven't been to an ENT, I'd recommend going, as there might be a physical blockage creating a pocket of infection.
If you have been to an ENT, they are correct, there is no definitive anti-fungal at this time.
A regular sinus irrigation won't work either, because gravity prevents getting saline to your upper sinuses. A Sinus flush will defeat gravity:
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground, or you can kneel down and lean over a bathtub with the shower running.
Would not recommend the peroxide either, as it can mess up the good bacteria balance in your sinuses and make it worse.
If there's no improvement after 10 days of sinus flushing, stop. Then you may have other issues, discussed on the following forums.
http://www.medhelp.org/posts/Respiratory-Disorders/Bad-smell-coming-from-noseFound-a-Cure-Yayyyy/show/520721
http://www.medhelp.org/posts/Allergy/Weird-Smell-in-my-Nasal-passage-will-not-go-away/show/1080376
Finally, search for an article with this title, to see what's going on in the world of anti-fungals. Search also for Jens Ponikau, he is the expert, so far.
Fungal Theory Debated in Amphotericin B Controversy
good luck