Ad hominem is not a fancy Latin phrase meaning "nasty insult", it's the name of a rhetorical flaw in which, rather than addressing a person's arguments, one criticizes the person making the argument. In other words, instead of saying that a member's argument is invalid, one says that he's an idiot or a liar (insert your favorite here). A rhetorically valid argument is valid, or not, NO MATTER WHO MAKES IT. Focusing on who's making the argument rather than on the argument itself is "argumentum ad hominem", i.e., arguing "to the person" ("ad hominem").
When one's criticism of an argument is that it was made by a "lying son of Satan" (for example), one is indulging in ad hominem. But ad hominems are not necessarily nasty insults. It is ad hominem to say that a person's argument is invalid because they are a believer in a particular religion, a member of a particular political party, or any other personal characteristic. It is even, technically, ad hominem to say that an argument must be true because the person making the argument is saintly.
Bottom line: criticize ideas, not the people expressing them.