Larry,
One additional note: If you are using 4-wire chassis fans, it is possible that the fans will not stop spinning even if the duty cycle is dropped to 0%. The 4-wire fan specification allows for fans that do not go below certain speeds, etc. I would consult the datasheet for the fans that you are using so ascertain its method of operation.
You will not see this issue if you are using 3-wire fans...
...Scott
P.S. You can read more about the possible modes of operation for 4-wire fans within the 4-Wire Fan Specification, which is available at the http://www.formfactors.org web site. Search the specifications using Category "Other"...