iplock
Easy to use tool to block unwanted network access.
...This is particularly useful if you have a webserver such as Apache2 running a scripting language such as PHP without root permissions. When you detect that a user is likely a hacker (i.e. tries to access a folder named wp-admin on a non-Wordpress website) then you can use iplock <his-IP-address> to block him.
The iptables commands are defined in the configuration file so you can really tweak your firewall to your liking.