I make a distinction - and perhaps you do as well - between dismissing a claim or argument because its source is considered unreliable, and concluding that the claim must be wrong if it came from such a source, only the latter being the informal ad hominem fallacy, the former being sound judgment.
So if somebody with a reputation for lying makes a claim, even if followed by evidence, argument, and conclusion. I feel justified in ignoring it all.
One may offer that the argument should be evaluated on its own merit, but the problem there is that to do that properly, one needs not only to fact check the claims of fact in the argument, which is hardly worth the effort in most cases, but also perform the much more difficult task of trying to identify what evidence has been omitted.
Consider this bit of creationist apologetics from DNA tests prove Darwin Was Wrong - Ape DNA very different from human DNA - Laws of Genetics Contradicts Ape to Human Evolution
"But the fact is, humans actually have 23 pairs of chromosomes while apes have 24 pairs of chromosomes. Apes, gorillas, chimpanzees, orangutans, etc. all have more chromosomes than humans. During the first half of the 20th century, that fact would have seriously weakened "ape into human evolution theory" because there is no way to explain how apes, with 24 pairs of chromosomes, could have evolved into humans with 23 pairs of chromosomes. We all know that if we lose a pair of chromosomes, we cannot reproduce."
How can you adequately evaluate that argument without already knowing that human chromosome 2 has the morphology of two chromosomes merged end-to-end, a fact that is actually considered a smoking gun for confirming the claim that all existing great apes arose from a common ape ancestor? Without that fact, the argument appears sound. That is the problem with saying that the argument should be evaluated on its own merit when its source is known to be disreputable.
Incidentally, does anybody believe that this source isn't knowingly omitting relevant evidence to make a dishonest case? Not that it matters, because ignorance of the fact of human chromosome 2 disqualifies a source contradicting the consensus of the scientific community as quickly as dishonesty, and constitute a second reason not to take one's science from such sources.
But notice once again, the claim is not that the argument is unsound because it comes from such a source, which would be the genetic fallacy, but that the argument isn't even worth considering if the source can't be trusted to be arguing sincerely and in good faith, since a much larger investment in time and energy in reviewing the topic is needed to see what claims are false and which facts have been omitted. The claim is that the untrusted source's argument or claim isn't worth the effort to investigate, which is not a logical fallacy, but a personal judgment.
What I will generally post in response to being offered a link to Answers in Genesis or Old Earth Ministries is a request for the same material from non-creationist site, noting that if the claim is true, it was imported from a legitimate information source that should be acceptable to both of us, and to please cite those sources instead.
The obvious corollary to this is that if the material can only be found on a creationist website, it's probably science fiction, since nothing that is true is known only to creationists.
In the end, saying that one doesn't trust a source and rejects its claims is not a logical fallacy as long as one doesn't go so far as to say that the claim is false before evaluating it.
