Ad hominem abusive
Ad hominem abusive or abusive ad hominem attacks are a subset of ad hominem attacks. An ad hominem attacks the person making the argument or the source of the argument rather than the argument itself. Abusive ad hominem attacks are insults directed at the source of an argument or counter-argument.
- "Why would you believe Bob's story? Bob's such an idiot, he walks into walls!"
This statement is fallacious because while Bob may have lost a few brain cells by walking into walls, it doesn't mean he's a liar, stupid or mistaken.
- "Nothing atheists say can be trusted because they're dirty lying scumbags."
This statement is fallacious because most atheists are usually truthful, are usually good people, and usually shower or bathe. Equating atheists with distasteful practices is a fallacy because it is usually untrue.
Incidentally Christians have at some times in history seen parasite infestation as a mark of saintliness, St. Francis was a notable example.
Insects," i. e. lice, were an unfailing token of medieval sainthood. 
- "You are utterly ignorant of evolution, so I should not take your views on biology seriously."
This is not an abusive ad hominem attack because it has serious bearing on the quality of the argument being put forward. If one has no knowledge of the current scientific theory of evolution, it follows that they likely have no knowledge of the rest of the field of biology.
- "You're an idiot. Obviously, the God of the Bible is a complete prick, and it's demonstrated in the Old and New Testaments."
This is not an abusive ad hominem attack because although unnecessary verbal abuse was used, the argument remains valid.