The first thing you need to do is find out the IP address of the banned user.
This will list the current rules that exist in iptables.
You’ll see one listed as fail2ban-ssh, likely with an ip address.
To clear it, issue the follow command.
iptables -D fail2ban-ssh -s <ip_address_to_be_set_free> -j DROP
I have used the code above with Debian wheezy successfully.