Restrict Community User Access to Cases (Generally Available)
Where: This change applies to all communities accessed through Lightning Experience and Salesforce Classic in Enterprise, Performance, Unlimited, and Developer editions.
Why: For instance, your company has multiple offices but wants to keep its cases private within each office location. By disabling the standard sharing behavior, community users who transfer from one location to another don’t automatically retain access to their cases.
How: From Setup, enter Sharing Settings in the Quick Find box, and then select Sharing Settings. In the Other Settings section, deselect Grant community users access to related cases.