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.