I can definitely help you with that ....
On the conventional side of things, there are many things you can do to block
proxies from accessing your system ranging from bad IP lists to active proxy
server detection.
On the less common side of things, I have special technology I developed
that lets me see the real original IP address of all connections irregardless
of how many proxies or proxy chains they are piped through and that also
includes those behind so called "high anonymous" proxies and VPNs
of course effectively rendering privacy on the internet effectively dead.
I have begun licensing my technology to a number of select hosts already
so that would be another avenue you could look into.
What kind of forums do you have? Some forum software such as phpBB2
is better suited for defensive anti-proxy modifications which is yet another
layer you can add to protect from people trying to bypass bans.