Blacklisting an IP address in Linux which in turn will 'firewall' out that IP is nice and simple using the built in application called "IPTables".

To add an IP address to the Firewall/Blacklist simply do the following as root:

iptables -A INPUT -s 123.123.123.123 -j DROP

Replacing '123.123.123.123' with the IP address that you would like blocked. Removing an IP is just as easy, just type:

iptables -D INPUT -s 123.123.123.123 -j DROP

Again replacing '123.123.123.123' with the IP you would like removed from the list.

Just to make sure that IP Tables saves you changes, you may wish to type iptables-save as root which will make sure your additions or removals are stored.