This is probably going to **** Darren the Drunk no end, but I would think seriously of using some bleach!
Assuming its something like the common open head drum type fermenter. Depending on the exact material and how well the fermenter was made some plastics can be quite porous if you filled the fermenter wit hot (tap) water and added some bleach, shove everything made of plastic that you use in it (the lid will usually fit in if you push hard enough). Leave it to soak overnight. Next day, drain and rinse well. Put it in the sun for a couple of days, this will drive out most of the Cl. Then clean and sanatise as per usual.
Iodophors would be another good (better) option, the good old Pink Stain Remover (Chlorinated Tri-Sodium Phosphate) would work well. If the fermenter isn't in good condition replace it and all the fittings. Often if you get a sick fermenter its easier to just replace it than to try and get it sterile.
I know how frustrating persistent infections can be, sometimes its the environment, a tree in flower that is shedding masses of wild yeast, a vineyard over the hill, even a moldy dank old laundry that you shouldn't really be brewing in. Have a long hard look at your environment, if all else fails Stop take a short break from brewing, come back in a month or so, clean like buggery and try again.
Mark
Kadmium posted while I was typing - pretty much the same thoughts - you need to be absolutely scrupulous in your cleaning and sterilising.
Luck M