I think Andy Ihnatko's take (not on this issue, but on civility on the Internet[s]) is particularly germane:

http://ihnatko.com/2010/07/07/monitoring-the-ongoing-abrading-of-common-courtesy-update-822/

To wit:

You, personally, aren’t the most important person in the universe. You are the only important person in the universe.

There can be no excuse for anybody failing to adhere to your privately-held and poorly-defined expectations.

Never consider what life is like for anybody else. Filter everything that everybody else does through your own experiences and judge it against your random, ill-defined expectations of human behavior.

Remember: as the recipient of the right eye of Odin, you have all knowledge of all things, instinctively; trying to see something from another perspective is just a waste of your valuable time and patience.

If one of these idiots attempts to (oh for ****’s sake) defend their actions, you must react with either indifferent disdain or (better yet) outright hostility. Their so-called “explanation” can only be an attempt to bulls**t you. You’re too smart to let them trick you into double-guessing the validity of your immediate reaction to their choices; take immediate and violent offense that they even suggest that you’re not on to them.

By steering the discussion away from the topic at hand, and onto the subject of how much they offended you and how they must now make amends, you’re putting the focus right back where it belongs. See point one.