The Limits of Rational Discourse: When Do You Shut Someone Up?

Last Friday, the post was a meditation on race, looking at the loss of Dr. King and the changes that have occurred since. We had a horrible comment leading off the discussion and the responses were harsh.

I stepped in to stop the ad hominems and Kerry's response was that sometimes, that's the appropriate response. Reason is fine if the other side disagrees with you and is willing to be reasonable, but if they opt out of civil rational discourse, the rules no longer apply. If you have a bully, the line goes, they don't deserve the same courtesy you offer to those with whom you have a rational disagreement. the only appropriate response is STFU.

Is this the case? Or have you allowed them to bring the whole discourse down into the gutter with them? Is it better to show "intellectual class" even if it won't affect the bully, if for no other reason, for the sake of the forum? When do you silence people or should the conversation be left completely open to even the least well-thought out, over-the-line, anti-rational sentiments? At what point are you prizing tolerance to the point where it endangers tolerance?