To block an individual IP address from accessing your Linux server, you can use the following iptables command:
iptables -A INPUT -s IPADDR -j DROP
Simply replace IPADDR with the IP you want to drop all packets from.
Block ranges of IP's
Here is an example of a range block:
iptables -A INPUT -s 192.168.0.1/24 -j DROP
This would block 192.168.0.1-255
Unblock the IP
Simply change "DROP" in the commands above to "ACCEPT".