Ignore was added in a forum update for ALL the forums in March. I made a thread about it at the time with links to the WoW forums that had some screen shots. There are two ways to do it. One via your own preferences, and one from the avatar on the posting thread. It does work on the Classic WoW forums too. Just be sure to ignore the posting char there you want to ignore.
The latest update added the longer and perm duration on the ignore. Very nice.