There is no cure for respiratory diseases, only treatments. Sulfa drugs including sulmet treat coryza. The problem is that since the sulmet is mixed in water, sick birds wont drink, and if they do it's usually not enough to be effective. Additionally there's a possibility that mycoplasma gallisepticum might be a player as well. Coryza sick birds exude a foul odor around the head area. If there's no foul odor, you could be dealing with a severe strain of mycoplasma gallisepticum instead of coryza. Sulmet is ineffective against MG.
I recommend that you have a necropsy performed on your sickest bird with the University of Georgia vet med school to find out exactly what you're dealing with or contact your county extension agent. Your other option is to cull your sick birds, disinfect everything and wait several months before repopulating.
X2. I also believe a necropsy is in order here. Oxine AH is also a valuable sanitizer to always keep on hand around the barn and coop:
http://www.shagbarkbantams.com/oxine.htm