If you want to disable the option of adding a new post under a Custom Post Type in WordPress (3.0) then there is a meta capability create_posts that is used by WordPress to check before inserting the various 'Add New' buttons and links but this is not documented. You can see the code below:-
register_post_type( 'custom_post_type_name', array(
'capability_type' => 'post',
'capabilities' => array(
'create_posts' => 'do_not_allow',),
'map_meta_cap' => true,
));
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