Using CiviCRM 4.6.15 (and later 4.6.16) integrated in Joomla 3.5.1 I just found out that it is no longer possible to change ACL settings for CiviCRM. It is possible to change a setting, this change is ajaxified and will be shown immediately, however after saving the change it is rolled back to the setting before the change.
I see this in a site that I upgraded from an earlier Civi version, and also in an install of CiviCRM 4.7.6 in a Joomla 3.5.1 site. The readme text with the ACL settings advised to set the value of 'max_input_vars' (defult 1000) higher. I reset this to 2000 and later to 8000. This does not solve the issue with CiviCRM 4.7.6 (now 4.7.7) in Joomla 3.5.1 however.
This seems to be a longstanding bug, see: https://issues.civicrm.org/jira/browse/CRM-15841.