Remove unnecessary packages, CentOS 7, Virtualbox

yum -y remove alsa-* ivtv-* iwl*firmware aic94xx-firmware*

Install yum-utils:

yum -y install yum-utils

List packages:

package-cleanup --leaves

Remove packages:

yum remove `package-cleanup --leaves`

Clean up yum cache:

yum clean all

Remove cache dir:

rm -rf /var/cache/yum

Remove all old kernels:

package-cleanup --oldkernels --count=1

Related:
CentOS 7 Minimal Kickstart

Other clean command:

yum clean all && rm -rf /var/cache/yum && yum makecache fast