1) Login to the server as via ssh with root user..
2) If you are running iptables, you can enter:
iptables -A INPUT -s <IP> -j DROP
3) If you have APF firewall installed
apf -d <ip>
4) When you reboot this IP ban will be removed, meaning the IP will no longer be banned anymore.
If you have APF you can get around this by opening the deny hosts file.