It's best if you use su or sudo rather than allowing remote login as
root. However, if you are on a LAN with no connection to the internet,
you could:
remove all non-commented lines from /etc/securetty, or
add all /dev/pt* or whatever devices are used as the controlling tty
when you login remotely, or
modify the sshd configuration to allow root login, preferably using keys
and not passwords.