have you searched the Red Hat knowledge base ?
you are paying red for the required license and support contract ?
log in to red hat with your RHN credentials and do a search
in a normal install the "installmedia" repo text file SHOULD not be turned on ( or even in the yum.repo.d folder )
open it up and change all the "enabled=1" to "enabled=0"
then can you run this ( and if it works post the output)
------ your root password when asked for ------
yum repolist all