How to unregistered the server from the Red Hat Network

If the system is registered to Red Hat Satellite Manager run below commands

# subscription-manager clean

If the system is registered to Red Hat Network Classic method then remove or move the systemid file from /etc/sysconfig/rhn/systemid to some other location

mv /etc/sysconfig/rhn/systemid /tmp/
or
rm -rf /etc/sysconfig/rhn/systemid

Once the system is unregistered then clean yum cache:

# rm -rf /var/cache/yum/*

# yum clean all

This will remove the server from Red Hat Network and  remove redhat.repo from /etc/yum.repos.d/ as well.