Sometimes "couch philosophers" join in a debate.
Usually, these are the people who don't know shit on the subject at hand. Their knowledge is basically what they just came up with. Their authority is based on the fact that they've read, or heard, something, somewhere, at some point in time. Even if it weren't complete tabloid class bullshit, they've usually only skimmed over it, without true comprehension or understanding the context, forgot most of it, and filled all these blanks with whatever fitted their world view.
They enjoy indifference, since the topic in question doesn't really concern them, and they call it "an open mind". They don't try to understand the emotions of the other person, that is directly troubled by the issue at hand. All they care about is discussing for the sake of it. Sometimes they actually enjoy when the debate turns into a shitstorm, when they manage to trigger an emotional response, and they gloat in their "alpha male" "superiority".
There's no point in talking to these people. You need to recognize the symptoms quickly, and ban them. Every counterargument is giving them satisfaction that they've triggered an answer, that they've got everyone's attention on them. The best you can do is take away the possibility of further participation from them.