To disable package updates by ULN (for example, if you have deleted your system from ULN), edit the /etc/yum/pluginconf.d/rhnplugin.conf file, and change the value of enabled flag from 1 to 0 in the [main] section, for example:

[main]
enabled = 0
gpgcheck = 1

To disable updates for particular packages, add an exclude statement to the [main] section of the /etc/yum.conf file. For example, to exclude updates for VirtualBox and kernel:

exclude=VirtualBox* kernel*