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.
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
The user name and password is your POP account user name and password. This information is available with your IT or System Admin team. Note: Please do not give your Zoho account user name & password.