WordpressMu: Don’t allow new blogs

If you’re using WordpressMu, the blog hosting tool used on Wordpress.com, you may want to disable the creation of blogs by your visitors. Whatever your reasons for this are, I wanted to prevent this, because I (and my team of editors) want to maintain several blogs on different topics. Users are free to register and post comments, but creating new blogs is reserved for the administrator. So, how do you implement this in WordpressMu?