Ken
You have probably banned an ip range by mistake.
Comcast and qwest (and nearly everyone else) charge extra for static ip’s.
Even though you might keep the same one through successive DHCP release/renew cycles or even be attached to one via radius or some similar software at the isp, unless your paying for it, your ip can change easily and sometimes regularly.
You may have banned the ip of a mail server depending on what criteria you use and what software.
The mail servers in my ip range have been banned from Free Republic since 1999. :)
There are a dozen different ways to do it in php and several should be used before resorting to IP.
I am not sure it is still the case, but AOL used to send all traffic via a regional gateway proxy which meant that if you ban the proxy server IP, you have banned hundreds of thousands of users. If you ban the range that includes several dozen proxy servers you could ban millions of potential customers.
Just nuke the account. or log it in to a version that cannot post. You already have it set to allow only registered to post.
At the very least do a traceroute (tracert in windows) and see where the ip address goes.
Also check the “home” link on your front page. It points to your hard drive on your mac.
Did you know you share that server with 28 other sites?
A link to some nice generic tools of interest: