When your website means business.

Users | Type: PHPRemove Registration Link from Login Page

If you want to control how users register for a member site, it’s a good idea to remove the registration link on on the login screen.

/*remove registration link from login page* 
 add_filter('register','no_register_link'); 
 function no_register_link($url){ 
 return ''; 
 }

To hide both register link and lost password (somewhat dangerous):

/**
 * add custom admin login CSS to hide the Lost Password and Register links
 */
function isa_login_css() {
    echo '<style type="text/css">body.login #login p#nav {display:none;}</style>';
}
add_action('login_head', 'isa_login_css');