The Symantec Management Agent (SMA) will not delete using both the "AEXAgentUtil.exe /uninstall" or "AEXAgentUtil.exe /clean" commands.
ITMS 8.7
Go to the following directory on the computer from which you want to delete the Symantec Management Agent:
C:\Program Files\Altiris\Altiris Agent
and run the following command line:
AeXNSAgent.exe /uninstall
NOTE: Also, make sure you have the latest version of the agent installed and consider upgrading to the latest pointfixes.