It's not totally ad hominem... I know the guy, and I know that he can make a compelling-sounding argument against something that I know (believe/whatever) to b

It's not totally ad hominem... 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.