In WP older than 3.6 it´s possible to deactivate or limit revisions in WordPress with the entry

define('WP-POST_REVISIONS', 100);

It seems that it is not working any longer in WP 3.6. Fortunately there is a nice little plugin that works very reliable and is able to delete a whole bunch of old revisions between two dates:
http://wordpress.org/plugins/thin-out-revisions/screenshots/