How to disable "Plain text" for a specific role? I see that from admin/config/content/formats/plain_text I can not disable it for a individual role. How can I do this?

UPDATE:
I can change the attributes of the format and rename it, however I see no way to change the machine name of the format. It should be a better way to achieve this.
Better Formats module will allow you to select the default input format per role, and disable the formats selection part of the node edit form http://drupal.org/project/better_formats