Disable revisions for a specific post type only?

Remove the revisions property inside the supports parameter of register_post_type().

Example

$args = array(
    // ...
    'supports' => array('title','editor','thumbnail','comments','revisions')
); 
register_post_type('book',$args);

Change to:

$args = array(
    // ...
    'supports' => array('title','editor','thumbnail','comments')
); 
register_post_type('book',$args);