iptables -A OUTPUT -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
iptables -A FORWARD -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
You can also block outside connections that attempt to spoof private IP address ranges to infiltrate your LAN. For example, if your LAN uses the 192.168.1.0/24 range, a rule can set the Internet facing network device (for example, eth0) to drop any packets to that device with an address in your LAN IP range. Because it is recommended to reject forwarded packets as a default policy, any other spoofed IP address to the external-facing device (eth0) is rejected automatically.
iptables -A FORWARD -s 192.168.1.0/24 -i eth0 -j DROP