I have used a substance called "Spray Nine" which removes tough stains. It seems to do the trick most of the time on these fungus stains on the surface of my house. It is by Knight Marketing Corporation251 North Comrie Ave, Johnstown, NY 12095 Ph: 518-762-4591
I have also read where Delco Cleaning Systems of Fort Worth in an article called "Shotgun or Artillery Fungus, Mold and Mildew by Bret A Lambdin said that to remove it from a surface I have read where you can use a pressure washer at 3000 psi with hot water at 200 degrees. If you can catch it soon after it has attached to a surface there is a better chance of it coming off (2 to 3 weeks max). The longer it is on a surface the harder it is to remove. Also, that they recommend using only non-chlorine based cleaners in conjunction with coating which contains an EPA registered fungicide, such as Wolman F&P. see- www.dcs1.com/articles/shotgun.html
Hope this helps.