Fail2ban should ban an IP address, that tries to register with false password or extension.
What ports do you have open?
When forwarding ports you should always only allow IP addresses that you know (whitelisting) through your firewall.
What firewall are you using?
Banning certain IPs (blacklisting) is not enough. There are plenty of machines out there just scanning your firewall for open ports and you will again see unwanted registration attempts from unknown IP addresses.
The best security practice for managing your PBX remotely is not to forward ports at all, but run a VPN server that you connect to.
Please read the two links below about recommended security practices with Asterisk and FreePBX.
http://nerdvittles.com/?p=580