CentOS 7. Избавляемся от мусора в /var/log/messages (Created slice..., Stopping User...)

Создаем файл garbage.conf в /etc/rsyslog.d/ с таким содержанием:
if $programname == "systemd" and ($msg contains "Starting Session" or $msg contains "Started Session" or $msg contains "Created slice" or $msg contains "Starting User" or $msg contains "Removed slice User" or $msg contains "Stopping User") then stop

Перегружаем rsyslog:
# systemctl restart rsyslog