The term 'ad hominem' is Latin for 'about the person'. This class of fallacy refers to comments about the opponent that are intended to either convince the opponent to stop arguing, or to convince the audince to stop listening to his arguments.
An abusive ad hominem commonly involves some sort of insult, and is usually pretty blatant.
"My opponent, the idiot, should not be tolerated in good company, much less paid attention to."