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.