How can I set that only root kan login to my SSH?

You shouldn't do that, or explain why you would do this. Getting in as an unprivileged user and using sudo (preferred) or su to get to the root account is "better". For example one could try passwd guessing for the root account. If you think this is bogus, as a test try running John the Ripper on your passwd file with a sufficiently large dictionary.
The sshd_config var btw is "PermitRootLogin".

If you want to have certain users from certain addresses be able to login, you can use TCP Wrappers, if sshd is configured with libwrap, or block addresses in your firewall, or use the (Allow|Deny)(Groups|Users) directives in sshd_config, or add a PAM module like pam_listfile to your PAM ssh stack.

This is a potential security hole like unSpawn pointed it out. A system compromise using SSH as source is a welcomed way these days. Think of the SSH1 attacks, etc!

I'd never allow ANYBODY to login as root over a insecure network such as the internet. Root should be only allowed to login at the console (if at all)! Starting SSH using xinetd is a nice way btw to set up additional protections ...

Is it possible to permit root log in to ssh on the local network but disable root log in to ssh for external networks? (internet)