In Centos 7 there is no use of /etc/inittab file, instead we can use targets, because systemd no more uses the /etc/inittab file. If you want to change your current runlevel you must use systemctl command. So you can use the below command to temporarily change your run level.
systemctl isolate runlevel1.target
And use the below command to permanently set your runlevel
systemctl set-default runlevel1.target