From any card (say 'Home'), select the options tab, select the read setting.

Click Delete.  It throws an error now.  It should give a dialog that  you are deleting the only rule that applies to this card.  More generally, if a rule is a current permission rule for any card, you shouldn't be able to delete it.

Apparently, I've broken my site by trying this out.  You can no longer view http://gerry.wagn.org/Home when not logged in.  I think I'll try a cache-reset to see if that helps.