Usually, I try to respond when challenged. In this case, however, the dude was so far off that I suspected a Mormon respondent trying the old tactic of trying to split Catholics and Protestants by starting an argument over doctrinal differences.
This is usually a sign of desperation. If you can't reply with facts, then: change the subject of the debate, call the opponent a name, or dismiss the entire discussion with a meaningless term such as "anti-Mormon."
Sorry for the inadvertent omission.