To de-activate Customer Portal User, Goto Setup > Customer Portal Settings > Customer Portal Users. The list of Portal Users who have been added would be displayed. Select the user you intend to de-activate and click Deactivate Portal User.
Yes, you can re-invite your Customer Portal user if they have not accepted their first invite. To carry out this function, Goto Setup in Zoho Support > Customer Portal Settings > Customer Portal Users. In the Customer Portal Users page, you can find ...
No, currently it is not possible for a Portal User to view Requests related to other customers. However, a portal user can add requests and track his/her request status in customer portal. For more refer Portal User Action