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.