Hi Govind,

I am not sure if it will help or not but all i know is , Devices for notification preferences for all users stored in cmn_notif_device table . Thus un checking active for all devices should disable the notifications but not all notifications. if a notification is configured to be send to a user the system first check for the primary notification device of type email and if its not found then it will send email to the email field in the user record. You could inactivate all records using script but remember to keep   a backup. There will be some notification devices which are already inactive and if you are inactivating all the email devices then you will lost the record that which were inactive earlier.