Limit the number of posts that your authors can publish. This plugin by http://www.wpsite.net allows you to limit the number of posts that can be published based on role (i.e, author) or user.
It allows for the following:
- Limit number of posts by role (i.e., author)
- Limit number of posts by user (i.e., John Doe can be limited to 5 posts)
- Posts submitted after user has exceeded their limits will have a new post status of 'Limited'
- Limit number of posts by post type (coming soon...)