Hello,
Have you tried the Symantec Articles provided below:
Norton AntiVirus Corporate Edition or Symantec AntiVirus Corporate Edition prompts for password when uninstalling clients
Symantec System Center password management in Symantec Client Security 3.x and Symantec AntiVirus Corporate Edition 10.x
Uninstalling Symantec AntiVirus 10.x or Symantec Client Security 3.x silently
Password recovery for Symantec Endpoint Protection and Symantec AntiVirus
Using the "Email Tools Uninstall" utility
'Manual uninstallation documents for Symantec Client Security products'