This could be the controlm server buffer issue.
Refer to article for fix.
We are on v8.0.00.500 of the EM and will be upgrading to v9 in the coming months. I modified the "CmsHostSpecRetrieverInterval" parameter to "0" as suggested in the KA and will confirm Monday that they don't reappear. Thank you!
The first link provided solved the problem in v8. After updating the "CmsHostSpecRetrieverInterval" parameter, the agents have not re-appeared. Thanks so much.
Other way to eradicate this issue is.
If your Control-M Server is in Windows box please run "ipconfig/flushdns" in command prompt and then recycle Control-M Server process and for unix box please do restart nsdl and recycle Control-M Server processes.
Instead of directly deleting the failed agent, please do disable the agent first from CCM and then delete.