CentOS follows the upstream source in this respect, as it does in general, and the x86_64 installation by default will install iX86 32-bit packages on a 64-bit installation for compatibility purposes. Many server system administrators (and some desktop users) want a pure 64-bit system and so remove all 32-bit packages. This can be accomplished as follows:
yum remove \*.i\?86
To keep any 32-bit packages from being installed in future updates, edit your /etc/yum.conf and add the line:
exclude = *.i?86
Be aware that 32-bit applications, including some browser plugins that may only be available in 32-bit versions, will no longer work after this procedure.
You may also want to do this:
yum reinstall \*
The reason is that sometimes the /usr/share/ items (shared between BOTH packages) get removed when removing the 32-bit RPM packages.