Remove the More Link

You need a filter on the_content_more_link.

else { // They aren't in the paying group  
  function strip_more_link($link,$linktext) {
    remove_filter('the_content_more_link','strip_more_link',1);
    return $linktext;
  }
  add_filter('the_content_more_link','strip_more_link',1,2);
  global $more;
  $more = 0;
  the_content('<b>Interested in reading the rest of this article? You  just be a subscriber to continue reading.</b>'); 
} 

It may be more efficient, or neater, to write a check into the callback rather than add and remove the filter inside the Loop but I don’t how your “paid” vs “not paid” subscribers are marked.