How-to block registered users from access to WP backend on multi-site network?

One site on my network is able to block users from accessing dashboards on other sites in the network, but the other 2 sites allow users to type:

http://sitename.com/wp-admin

And have access (limited access), but access nonetheless to a dashboard.

I'm using:

http://wordpress.org/extend/plugins/multisite-user-management/

I had the plugin set new users' role to contributor but have now changed all new roles to subscriber. Folks can still access wp back-end dashboard on the 2 sites mentioned.

Where can I block access to these sites in settings or elsewhere?

thanks in advance,

Elmer (Brand-new WPMU Dev member)