CMS Prevent users from editing articles after they're published
 

How can I prevent users from editing their articles after they're published?

On my site, I'm planning to have a section where users can submit articles to the CMS. After they're submitted, they'll be reviewed/edited by a moderator and then published to the appropriate section. After this has been done, I don't want to the user to have the ability to edit their article anymore (otherwise they could edit their article to put arbitrary content on the site's main homepage).