I don't think there is a single word for this phenomenon itself. The closest term for the respondent is "narcissistic rage". Perhaps "narcissistic groupthink" for the larger phenomenon of making the OP a heretic or pariah.
As to what you can do, most internet forumgoers don't seem to have a very deep recollection of the past misdeeds of others; as cruel and shallow as commenters get, they're also faster to forget. Unless you dwell in a small or focused community, pretending it never happened is the best way to move forward- preferably shifting your time to a more enlightened forum, or, better yet, coming up with the next big thing.
As to what you can do, most internet forumgoers don't seem to have a very deep recollection of the past misdeeds of others; as cruel and shallow as commenters get, they're also faster to forget. Unless you dwell in a small or focused community, pretending it never happened is the best way to move forward- preferably shifting your time to a more enlightened forum, or, better yet, coming up with the next big thing.