An ad hominem is when you attack the person (such as by bringing up their previous comment history) rather than the substance of their argument.

Who the poster is and what they've said in the past are completely irrelevant.

Good rule of thumb: Would the rebuttal remain the same if responding to a different poster? If no, then the rebuttal is probably an ad hominem.

Yes, if that different poster believed the same thing.

There is a whole army of people out there who have a serious problem with accepting reality and it is useful to point out that someone is arguing with someone else who does not accept reality as it is commonly experienced. That saves everybody time, just like we don't feed the trolls we also should not waste cycles on engaging people who refuse to accept evidence anyway. Think of it as a PSA rather than an ad-hominem attack, you are assuming that the argument was brought in good faith, I don't.