MODULENAME_form_alter(&$form, &$form_state, $form_id) {
if ( $form_id == 'comment_form') {
  global $user;
   if (!user_role('Premium'){
    // Form alter here to unset comment form and show "only premium members can comment"
    unset($form['author']);
    unset($form['subject']);
    unset($form['comment_body']);
    unset($form['actions']['submit'] );
    unset($form['actions']['preview'] );
    print "Only Premium members can comment";
  }
 }
};

function MODULE_form_alter(&$form, &$form_state, $form_id) {
    global $user;
    $node = $form['#node'];
    // a custom function to do your logic
    if (comment_should_be_restricted($user, $node)) {
        // unset form elements, you can fetch them easily by print_r(array_keys($form));
        unset($form['comment_body'], $form['actions'], $form['author'], $form['subject']); // etc.
        // add a optional message for user.
        $form['no_access'] = array(
            '#markup' => t('You have no access to comment on this content.')
        );
    }
}