Sometime I needs to install an older flavour of a package.  You can list these with yum and install a specific version, e.g.

yum --showduplicates list mod_ssl
yum install mod_ssl-2.2.3-53.el5

I wanted to run all updates from the RHEL repositories, but not the rpmforge and WAN Disco ones. Easy.

sudo yum check-update --disablerepo=rpmforge --disablerepo=WANdisco-dev