WillyNilly makes some good points, and I thank her for using X-files as an example, because it's a really good one.
Elephantschild, a lot of your post actually brings to mind the age old question about how much does a creator of a series "owe" to their fans. Say, Star Wars. But, I know those things are a little bit to the side of what you were actually asking. Which is about how to deal with the forum fallout.
As a personal rule, if I'm on a forum dedicated to a show or comic, I feel it's fine to go into a thread or create a thread that constructively criticizes a plot line. For example, "I really disliked that Character kissed her ex-boyfriend. That was absolutely against what they said just 5 minutes before. I think they could fix it if they did XYZ. Or, here's why that doesn't fit Character so far." Things of that nature.
However, let's say the plotline continues on and that Character ends up having a new relationship with ex, and there are threads springing up discussing NOT what they think about that, but rather where they see the plotline going, then sure I can mention that I'd like to see the plotline concluding with Character breaking up, but I don't get to sit there and call the creators traitors or anyone who likes the plotline a traitor, or make fun of people for liking the plotline.
Really, if I'm that angry about a plot choice, then I'm probably going to be handled better by a creator if I write a polite letter detailing points than calling them names on a forum and hijacking every thread dedicated to that one specific plotpoint.
But, forum etiquette is largely dictated by "how much can I get away with". The moderators are the final line. If they choose to let something go after it's been reported, then they are saying that "these are the rules on this forum, that person is allowed to do what they are doing". If they choose to arbitrarily follow rules based on who posted or even what mood they're in, then that, too, is saying, "Sometimes the rules are followed, sometimes not. That's the way it is on this forum."
PS - I'm curious what entertainment series you're talking about, but I don't think it would change my mind, as it's eye-rollingly juvenile to call a creator a traitor to a series.
Adding a bit: If the moderators are allowing the rants or whatever they are, then a person can either choose to put up with it by (as WillyNilly mentioned) not reading certain people's posts/blocking them, or by finding a different forum that more suits their needs as a participant.