Ad Hominem fallacies occur when an attempt is made to discredit the person rather than addressing the evidence. Ad Hominem attacks are very common. They consist of the claim that something must be false because the person who said it is not thought to be credible, regardless of the argument itself. The only time it may be rational to question the person is when the person alone is the premise for the argument. When someone uses authority as a premise, that authority can be brought into question.
Tip: When someone attacks you personally, realize that they have run out of gas. They have nothing of substance to say.
"Did it ever occur to you that you are a crazed religious nut?" (and therefore I am not going to listen to a word you say, or try to refute your arguments).
"I'll bet you get your information from [insert a person or website here]. They are not believable. Everything about them is wrong."
