I was chatting with someone today about how online "discussions" happen, and I realized that there is a fairly clear cut way that debates on any subject degrade. This is your handy guide to navigating these troubled waters.
When you start to run out of good points, try to shape the argument and make it about something else entirely. While some complex subjects certainly have peripheral issues to discuss, most of the time I'm talking about wholesale derailment. If not elephant, then chair or burrito. Everyone likes burritos, so there's a solid chance you'll be right in the end.
Then there's the classic, "You 'obviously' didn't read my post," defense. This is best used when you're absolutely sure that, regardless of the response, you're still right in the first place, and the other person is "obviously" just stupid. Be sure that if you use this, that you at least know the difference between "there," "their" and "they're," or you may end up looking more stupid than the jerk who didn't read your post.
Perhaps you're just annoyed that you're not getting anywhere, or getting shot down at every turn. That's when you start to make it personal. I mean, if you disagree, then you're just a socially inept know-it-all, or you live with your mother, or smell bad. (The last part is actually why Mac users are always right, because using a Mac does in fact make you smell better.)
In the case that you're debating with the person who runs that particular forum, or a moderator, then you bust out the, "You're picking on me," move. If you're not in charge, you automatically have a disadvantage, right? If things get too out of control, well, that's when you invoke Godwin's Law and enjoy the fantasy that you being put down in a forum is exactly like millions of Jews being slaughtered.
Using these trusty rules, you can endure and persevere in any online forum.
I remember when "debates" like that were more prevalant in the winter months and much less during the warmer months because people were simply outside doing things rather than cooped up with nothing better to do.
Hell, I remember a time when you could have a really informative and challenging debate on line...but I'm an old fart and "netiquette" has gone to hell now.
At least once you pick the worst offenders out, you can totally ignore their posts, but it's a damned shame when a good debate goes down the tubes because of actions you described.