Block IP address using iptables

 

iptables -A INPUT -s [IP ADDRESS] -j DROP

For example:
iptables -A INPUT -s 1.2.3.4 -j DROP

You may have to look under /sbin for the executable

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To save iptables rules / config use:
iptables-save > my-rules.fw

To restore iptables rules use:
iptables-restore < my-rules.fw