How do they see user profiles? Do you mean being able to see the names of the people who recently registered, posted, etc? You can turn off their ability to actually view a profile (or any file for that matter) by going into the corresponding script (showprofile in this case) and changing this:
"regonly" => 0,
to this:
"regonly" => 1,
They'll be able to click on a profile name, but will get an error that they don't have permissions.