Do you really need to ask? Ok, say an atheist wrote a book about the truth of Christian Dogma, and a Christian said "Don't read it, the author lied on their taxes", could you figure out who was posting a fallacy? Or is it the definition of ad hominem?
Just in case:
Argumentum ad hominem literally means "argument directed at the man";
The first is the abusive form. If you refuse to accept a statement, and justify your refusal by criticizing the person who made the statement, then you are guilty of abusive argumentum ad hominem. For example:
"You claim that atheists can be moral -- yet I happen to know that you abandoned your wife and children."
This is a fallacy because the truth of an assertion doesn't depend on the virtues of the person asserting it. A less blatant argumentum ad hominem is to reject a proposition based on the fact that it was also asserted by some other easily criticized person. For example:
"Therefore we should close down the church? Hitler and Stalin would have agreed with you."