You can use the Quick Actions 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 below:

1. Navigate to Explore Sites and click the Quick Actions button:
   
   
   
   
2. Select the second option in the wizard: Clean Up Orphaned Users.
   
   
3. Click Next, and review the list of Orphaned Users:
   
   
   
   
4. Select the users you wish to clean up, then click Remove Permissions.
   
   
5. 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 Activity Log, along with the details of the user account that performed the action.