I wonder if in the Web.config file you could explicitly revoke login  for all users, with something like <deny users="*" />.

Using the example from the following MSDN link:http://msdn.microsoft.com/en-us/library/8aeskccd.aspx

<configuration>
  <system.web>
    <authorization>
      <allow roles="admins"/>
      <deny users="*"/>
    </authorization>
  </system.web>
</configuration>