You could just ban the user as well if you don't want them in there. It's a very useful feature and has many uses other than "encourages a poorer atmosphere on IRC".
Your definition of a "poorer atmosphere on IRC" would differ greatly from mine, and mine would differ from the next guy. But in the end you can disable it, you have the power to stop a user from auto-rejoining, and you can let others use a feature that they've used for many years.
