Disable, Limit, or Delete Post Revisions in WordPress

To disable post revisions add towards the top of the wp-config.php file:

define( 'WP_POST_REVISIONS', false );

To limit the number of post revisions (i.e. 3 revisions) add in wp-config.php:

define( 'WP_POST_REVISIONS', 3 );

To delete the old revisions, do an SQL query in phpMyAdmin SQL interface.

DELETE FROM wp_posts WHERE post_type = "revision";

Be sure that you have replaced wp_ with your own WordPress database prefix.
A database backup before this query execution is highly recommended.