$ iptables -A FORWARD -p tcp -i ethX -s X.X.X.X --dport 25 -j DROP And this will immediately stop all traffic from IP address X.X.X.X coming *IN* to ethX on your firewall.
$ iptables -A FORWARD -p tcp -i ethX -s X.X.X.X --dport 25 -j DROP And this will immediately stop all traffic from IP address X.X.X.X coming *IN* to ethX on your firewall.