And many times its required that when you run the system/package updates you don't want to upgrade certain package and for this just add an exclude line to the end of your [main] section in /etc/yum.conf, and you are done. Your yum.conf should look something like this:
[main]This will prevent upgrades of the gdm and grub. The list of apps in the exclude should be space sepeated. Shell wildcards ( * ? ) can also be included.
cachedir=/var/cache/yum
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
installonly_limit=3
exclude=gdm grub
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source:http://linuxpoison.blogspot.com/2009/04/13578175711110.html