There are many ways to uninstall the Altiris Agent on managed computers and two of those methods are:
1) Policy method: Settings -> Agents/Plug-ins \ Symantec Management Agent \ Windows \ Symantec Management Agent for Windows - Uninstall. Configure the target for apply to computers with Symantec Management Agent installed.
2) Push method: Settings -> Agents/Plug-ins \ Symantec Management Agent \ Symantec Management Agent Install. On Rollout Agent to computers, click on Select Computers and add the target computers with Altiris Agent. Also is posible to import computers to a selected file. After these steps you could use the Uninstall button to start the uninstall process.
Regards,