fail2ban not working
Something else that occurs to me is iptables. You must be actively using it as your firewall as fail2ban creates a temporary drop rule for the bad source address in /var/log/secure.
If you've done a CentOS 5.3/ISPConfig3 setup, by default the iptables firewall is off. In ISPConfig3 there is a Bastille version which I would recommend that you don't enable as I find it too restricting. Instead, I enable the CentOS version of iptables. The one that creates /etc/sysconfig/iptables rules.