That is to say, that even though he's banned from the server, he's still able to access the forums without issue. The proper method for appealing a ban is of course to not post a thread about it, but instead for the banned player to start a private conversation on the forum with the staff member that performed the ban, civilly explaining their case and providing any relevant evidence that might help them achieve a favorable ruling on their appeal. It's important to note, however, that appealing a ban even in the proper manner does not guarantee that the player will be unbanned before the prescribed time period expires (if there is an expiration). If the staff member denies the appeal, that's the end of it. Further pestering or becoming uncivil can likely result in an extension to the ban, possibly even making it permanent.
