You'll never be able to 'stop' it as long as you have a vinyl awning. I don't care how clean you get it, it'll still mold and mildew---unless you roll it out, clean, and dry it very frequently. The above posts tell you how to clean off the mold/mildew, not stop it. I had one on 3 different trailers and am meticulous about keeping mine clean and garaged and they always had smelled and looked bad. The way I finally 'stopped' it was to have an Acrylic Awning put on my last trailer; they don't mold/mildew.