Ad hominem would be if shimman had said something like "don't post rebuttals from people who are stupid meanyheads". Identifying a characteristic of the posters that affects their incentives is a perfectly legitimate reason to discredit their posts, or at least call their impartiality into question.
Ad hominem (Latin for 'to the person'), short for argumentum ad hominem ('an argument to the person'), refers to when a speaker attacks the character, motive, or some other attribute of the person making an argument rather than the substance of the argument itself.
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That's not an ad hominem, though?
Ad hominem would be if shimman had said something like "don't post rebuttals from people who are stupid meanyheads". Identifying a characteristic of the posters that affects their incentives is a perfectly legitimate reason to discredit their posts, or at least call their impartiality into question.
From wikipedia:
Emphasis mine.