I don't think there is a way to kill the spores that remain in the environment. It's suggested to not even compost infected plants because the spores will contaminate your compost. Fungicides can help control it during the growing season.

http://www.ext.colostate.edu/pubs/garden/02902.html

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7406.html

http://www.clemson.edu/extension/hgi.../hgic2049.html