First of all, my apologies to Deadmeat for taking out on him something he didn't necessarily deserve. I won't say any more about that, because that may just start an argument in a different quarter, and the purpose here is to get rid of that.
As many people have already said, there is no way in which you can stop flamers and trolls from existing. They are used to saying pretty much whatever they like with little fear of retaliation; to paraphrase an earlier comment. they're not going to get smacked in the teeth by a monitor.
The best thing that can be done is damage limitation; you can't stop a single troll from posting, but what you can do is remind well intentioned people that "rising to the defence" of the addonmakers is only making matters worse; telling them after the fact not to feed the trolls is shutting the stable after the horse has bolted.
Now this is where I get argumentative. I'm afraid I can't see how banners merely proclaiming support for addonmakers is going to affect the well intentioned people who immediately rise to the bait of the real trolls. After all, in their eyes they are "supporting" the addon makers, right?
I suggest another type of banner, a compromise between the ones supporting the addon makers, and the "stamp" produced by Deadmeat. Again, it's been pointed out that Deadmeat's solution could just be a red rag to a bull, drawing attacks by trolls. You're challenging them.
You cannot command other people, but you do command yourself. My suggestion is a banner that states what you yourself do. for example:
"I support the addon makers which keep Operation Flashpoint alive.
If I believe criticism of an addon is warranted, I will do so politely, giving details where possible to prove my case.
(Here's the kicker)I will not post in response to attacks on an addon maker. to do so would only inflame a situation which is of no benefit to the community.
I will report to the moderators any post which I think is unduly offensive to any member of this forum."
Now if one person were to have a banner like this, it's very possible that Declarations 3 and 4 will draw attack. But if it were to be adopted by the community, who's the troll going to aim for?
Again, with these banners everywhere, a new member cannot fail to run into them. He sees immediately what the community expects of itself. If he wants to stick around, he'll want one of these banners himself.
perhaps they could be handed out by the moderators, when the new member reaches a certain post count?