Have you tried (and I know it sounds heinous) bleach? I had this as a kid, and my mum put some bleach (not much, I don't remember how much exactly...) in a bathtub, watered down heavily obviously with a full tub of water. Seem to remember that I took a bath every night for a week. (This was after bathing in something that a Doc prescribed, that smelled like tar/creosote and didn't work.)
I googled this, and apparently this home remedy still persists...http://www.tipking.co.uk/tip/3284.html
Don't sue me however if you don't have good results!