So, without further ado, my solution.  Just add this to your functions.php file:

function dashifen_disable_login_and_admin() {
   if(
      strpos($_SERVER["SCRIPT_NAME"], "wp-login") !== false ||
      strpos($_SERVER["SCRIPT_NAME"], "wp-admin") !== false
   ) exit("Administration disabled.");
}

add_action("init", "dashifen_disable_login_and_admin");

Obviously, you can change the name of the function to suit your needs; just be sure to change it both in the definition and in the add_action() call.  It could also be spruced up a bit.  Even a simple redirection to a specific page on your site that perhaps gives reasons why you’re disabling the administration of your site would be better than a fairly bland message, but it serves the purposes I needed it for until a better solution is presented.

I’ll now go back into my cave and post again in a few months.