I have a few machines that have been retired (or just died) before I could uninstall the Agent.  I have been trying to figure out how to remove the agent name from the list, but every time I do a search, all I come back with is how to uninstall it from the client machine.  Well today I finally figured out what I should have seen earlier.

If you right click on the computer name in the list on the Agents tab and select uninstall a task view comes up.  Click on "Uninstall Agents" and the "Enter Credentials and Reboot option" dialogue comes up.  At this point you have to enter valid credentials and choose a restart option.  When you hit ok, and the machine no longer exists you will get a message similar to the following:

Click on "Yes" and the task list comes back and removes the DPM agent from the database.  And that means no more ghost entry for that machine.