I don't know what you are referring to about restricting sending email to certain users. The activity feed preferences can be specified on a per-user basis.
Users in Alfresco can be enabled, disabled, or inactive. Disabled users can be enabled at any time, but inactive users cannot be reactivated on Alfresco Enterprise Edition without a special license from support, as they no longer count toward the per-user licensing. MNT-15038 explains how to configure LDAP to synchronize the enabled / disabled flag.
With regard to disabling users the short answer is no - see [MNT-12283] Need to be able to disable external users - Alfresco JIRA
For a longer answer that indicates that it might be possible, although it's not clear how to configure it, see [MNT-15038] Unexpected behavior when disabling Active Directory user (New feature to sync userAccountControl) - Alfresco… Richard Esplin might be able to help with that
Found the docs Synchronizing user account status | Alfresco Documentation