I confess - I’m a logician. I’ve seen my share of malice in fandom, but it never bothers me as much as irrationality does. The ad nauseum disturbs me because it is an attack on the possibility of rational debate in the area which “bores” the attacker. The ad hominem likewise bothers me, not because I’m the scapegoat, but because, again, the actual debate is tossed out the window when the debater herself is attacked.
I don’t mention fan foibles unless I’ve seen them more than once. I don’t link examples when the people involved are malicious, but if it’s just something an average fan might say or agree with, I will. Maybe not this time, though. Instead of linking, let me just rephrase one of the more frequent ad hominem arguments of fandom: A tells B, You’ve already made up your mind, and walks away.
It’s an ad hominem in the literal sense of the phrase, since it’s directed not at the point at hand but at the person (B) making the opposing argument. However, it’s not used (fallaciously) to establish the truth of A’s side, but merely to excuse A from further debate.
First of all, it’s meaningless to say that B has made up her mind. Believe me when I tell you, A wouldn’t be bothering with the argument if she hadn’t also made up her mind, at least provisionally. People who don’t have an opinion in the matter don’t get involved in debates.
A’s real meaning might be taken as, even if I proved my point, you wouldn’t accept the proof, but unless A is psychic and knows for sure what B’s reaction will be, this is just a baseless allegation. It takes quite a bit of argument to get down to another person’s fundamental irrationality, if she’s fundamentally irrational - I know, I’ve done it more than once. If you haven’t done the work, you have no right to impugn someone else’s rationality.
Of course, A is always free to walk away, but not to blame her forfeit on B. People are busy, and not everyone enjoys a good debate. Some people are culturally biased against certain styles of argument, considering them frightfully impolite. I’d guess the majority of fans either don’t like to argue or can’t argue their way out of a paper bag, which makes fandom a dull place for me. Even so, I never find myself cutting off an argument with an irrational opponent by saying you’ve already made up your mind - that is hardly B’s problem. If I have the time, interest, or a sudden fit of educational zeal, I will argue until it has become quite clear that B is a hopeless case. Then I just walk away, and I don’t look back.