Add the following key to your user.config:
<add key="RolesThatCanViewMemberList" value="" />
If you want to turn member list view back on for authenticated users (the default), you can remove the key or change it to this:
<add key="RolesThatCanViewMemberList" value="Authenticated Users" />
After changing user.config, remember to touch web.config to restart the application and read the new value.