The following command will drop any packet coming from the IP address 1.2.3.4:

iptables -I INPUT -s 1.2.3.4 -j DROP

or

iptables -A INPUT -s 1.2.3.4 -j DROP

(-I inserts into config, -A appends)

Use the following syntax to block 10.0.0.0/8 on eth1 public interface:
iptables -i eth1 -A INPUT -s 10.0.0.0/8 -j DROP

Use the following command to view:
iptables -L -v

Use the following command to save:
service iptables save