I am trying to unblock an IP from which I was doing some tests. I have followed the tutorials on the net:
$ sudo /etc/init.d/denyhosts stop
$ sudo vim /etc/deny.
You can do this in 4 commands. It automates the earlier answer from @Abdellatif with a python script, so you should thoroughly glance over the source before pasting these 4 lines into the command prompt (replacing IP_ADDRESS with the ip address):
sudo /etc/init.d/denyhosts stop
git clone https://github.com/rsprabery/unblock.git
sudo python unblock/unblock.py
sudo /etc/init.d/denyhosts start
It should work on all Ubuntu systems. And it's fast. And you don't have to edit any files. But, you are running someone else's script as sudo.