Crie o arquivo /etc/iptables/blocked.ips com a lista de IPs ou redes a serem bloqueados.

Como exemplo

192.168.0.0/24
200.122.53.11
74.67.1.213

E o script que será o responsável por carregar as regras.

#!/bin/bash

IPT=`which iptables`
SPAMLIST="spamlist"
SPAMDROPMSG="SPAM LIST DROP"
BADIPS=$(egrep -v -E "^#|^$" /etc/iptables/blocked.ips)

# create a new iptables list
$IPT -N $SPAMLIST

for ipblock in $BADIPS
do
   $IPT -A $SPAMLIST -s $ipblock -j LOG --log-prefix "$SPAMDROPMSG"
   $IPT -A $SPAMLIST -s $ipblock -j DROP
done

$IPT -I INPUT -j $SPAMLIST
$IPT -I OUTPUT -j $SPAMLIST
$IPT -I FORWARD -j $SPAMLIST