Sorry to say, but this is becoming a wide spread problem from what I have read. No use in trying to sue someone that put it down. They did nothing wrong. They wouldn't even be able to tell if the fungus was in the mulch or not. This is a real problem and much research is being conducted from what I understand.
I 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
Also, 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
Hope this helps.