Banned for inactivity? Hmmmm......
heh, I've never seen that happen. I've seen accounts disabled... but banning for inactivity is simply something that doesn't happen.
Either way it doesn't matter what the circumstances are. we don't help with that.
Additionally, the only way you can get back onto the site without getting a new ISP is to break their terms of service (their TOS will certainly list the various ways of "masking" or hiding your IP address as breeches of the contract) in which case they will ban you for breaking the TOS.
have you considered contacting the administration of the site in question?