If by draft, you mean “autosave”, you can consider using this plugin:
http://wordpress.org/extend/plugins/wp-feature-disable/
If you want to disable revisions, instead…try this:
define('WP_POST_REVISIONS', 'false');
You can put that in your /wp-config.php file and it should immediately take effect. Any previously saved revisions in your database will need to be purged. You can do it by running the following MySQL query in phpMyAdmin:
DELETE FROM wp_posts WHERE post_type = "revision";