Problem Scenario
You have some GKE standard clusters that you want deleted. You try to delete them, but they do not go away. What should you do?
Solution
Root cause: Kubernetes clusters are self-healing. They are acting as they were designed. Once the instance group is deleted, you will be able to delete the instances via the web UI or via Google's Cloud Shell.
Procedures
See this link How Do You Delete VM Instances from GCP That Pertain to GKE?