I know the guy, and I know that he can make a compelling-sounding argument against something that I know (believe/whatever) to be true (ie evolution).
Thus I no longer trust anything he says because I don't trust his motives or his arguments to be made in good faith.
I happily continue to listen to arguments made by people I have had previous disagreements with, but not those who I have evidence that simply ignore facts and scientific process. My experience tells me it's a waste of time.
There's a difference between that and an ad hominem attack.
First, I think you'd have a hard time finding anything good to read if you only stuck with writers who sound wrong when they're wrong. Second, Stein's anti-evolution thing is a religious belief and you'd be wise to forgive someone's apparently irrational thinking in that area. Religion is special.
(But anyway I'm fine with ad hominem judgments. They're useful.)
Just because the argument has something about the person himself doesn't make it ad hominem. Saying "He's a shitty dresser, therefore don't listen to what he says" -- that's ad hominem.
Past proof of his reasoning abilities (or lack thereof) are totally fair game, not ad hominem at all.
Instead of an AH attack, OP uses a commonplace (we all believe in evolution here on HN) as well as appeal to authority (in reverse).
Speaking clearly and having a strong education is a necessary but not sufficient condition for being intelligent.