I tried lot and also visited
http://www.zoho.com/crm/help/customization/custom-fields.html where Zoho mentioned: You can edit, delete and hide some of Zoho defined fields, but note that there are some fields that you cannot delete, remove, rename or change from mandatory settings. These Zoho defined fields are mandatory in Zoho CRM. For example, Last Name in Leads, Subject in Tasks etc. SO WHAT CAN BE BEST SOLUTION ?