"Calling someone a troll is an ad hominem. We aren't supposed to do that
in the IETF. At least, not publicly."[1]
No, it is not ad hominem. Ad hominem is an argument of the form 'Hitler was
a vegetarian, therefore vegetarians are NAZIs'
No, that's not true at all. Your example is a different logical fallacy.
"Hitler was a vegetarian. His idea is, therefore, wrong, because
vegetarians don't have good ideas," is an ad hominem argument --
attacking the person, rather than the content.
"This person is a troll, so we should pay no attention to what he
says," is, absolutely, an ad hominem argument. Whether or not we
absolutely prohibit ad hominem arguments is something I'll dodge for
right now. But let's be sure we understand the difference between
these:
1. "I don't like you, and your argument is wrong because [response
that addresses the issues]." This might include an inappropriate
personal attack, but it's not an ad hominem argument.
2. "I don't like you, so your argument is not worth listening to."
This *is* an ad hominem argument, whether or not it includes name
calling or other unflattering characterizations.
Barry