Please login as the user "ec2-user" rather than the user "root".
The secret is here:
# cat /root/.ssh/authorized_keys
no-port-forwarding,no-agent-forwarding,no-X11-forwarding,command="echo 'Please login as the user \"ec2-user\" rather than the user \"root\".';echo;sleep 10"
You can define user-specific sshd config in ~/.ssh/authorized_keys file