If you want to block a specific IP address from a server (say, they are attacking you):
iptables -I INPUT -s address -j DROP
To block an entire subnet:
iptables -I INPUT -s ipaddress/range -j DROP
Example, using 192.168.1.5
iptables -I INPUT -s 192.168.1.5 -j DROP
or, the entire 192.168.1.0/24 range
iptables -I INPUT -s 192.168.1.0/255.255.255.0 -j DROP
or
iptables -I INPUT -s 192.168.1.0/24 -j DROP