To delete a DPM agent which is showing in the DPM Console , you simply right click the agent in question and press uninstall.
Now press uninstall agents at the next window.
You’ll now have a Credential box, so just type your domain credentials and then the task will start to perform and will complete successfully and delete.
If you want a delete an Agent of a server that no longer exists, follow the above steps and you will get the following message.
Simply press yes and this will remove the agent from your console.
Hope this helps :)