You can use the [Access Manager] to revoke all permissions to the organization's sites for users who are no longer part of the Microsoft 365 Organization. This action is performed for all sites of the organization.

To do so, proceed as follows:

1. Navigate to [Access Manager] and select 'Clean up Orphaned Users':
   
   
    
    
2. Click Next, and review the list of Orphaned Users. Select the users you wish to clean up, then click Remove Permissions.
   
   
    
    
3. You will then be shown a summary of the actions performed. Click Done to complete the wizard.

Note: All actions will be audit logged and viewable in the Event Log, along with the details of the user account that performed the action.