Is it possible to un-register an AWS EKS cluster from Rancher? What if the cluster was imported and not created with Rancher?

You can unregister it from Rancher, but if it was imported, it will still exist in AWS (https://docs.ranchermanager.rancher.io/how-to-guides/new-user-guides/kubernetes-clusters-in-rancher-setup/register-existing-clusters#additi[…]usters) and you can remove the Rancher resources if you plan to keep running the lcuster outside of Rancher (https://docs.ranchermanager.rancher.io/how-to-guides/advanced-user-guides/manage-clusters/clean-cluster-nodes#removing-rancher-[…]egistered-clusters)

ok, thanks this is exactly what i was asking. BTW, I just imported an EKS cluster created with eksctl and the only option in the rancher ui is delete, this will delete it from Rancher, not from AWS EKS, right?

Correct. If it was not created in Rancher, it won't be destroyed when you delete it from Rancher

thanks