The following method has been shown by our Quality Assurance teams and in a few customer environments to be the best "sure fire" way to cleanly uninstall the Symantec Management Agent.
Run the following 2 commands. It should be noted that some time needs to be given between the commands, depending on how you are scripting it:
Both executables are located, by default at: C:\Program Files\Altiris\Altiris Agent\