O comando a seguir removerá as versões do kernel ALL , exceto para a que você está executando no momento:
sudo yum remove 'rpm -q kernel | grep -v 'uname -r''
e para o futuro, por 7.4. Configurando Repositórios Yum e Yum :
installonly_limit=value
…where value is an integer representing the maximum number of versions that can be installed simultaneously for any single package listed in the installonlypkgs directive. The defaults for the installonlypkgs directive include several different kernel packages, so be aware that changing the value of installonly_limit will also affect the maximum number of installed versions of any single kernel package. The default value listed in /etc/yum.conf is installonly_limit=3, and it is not recommended to decrease this value, particularly below 2.