It sounds like one of those unofficial web policing services that wrongly believe that your IP address identifies you uniquely. Someone else using the same ISP as you has been detected as a flooder and blacklisted, and if you happen to get an IP address in the same range as the one used by the flooder at the time, you're effecctively blacklisted as well. The workaround would be to reboot your router and get allocated a different IP address, and then you could hopefully contact the forumup staff and explain the position.