After running a command like:

kubectl delete my-namespace

and the command hangs indefinitely after accepting the deletion request, you might find it sitting at Terminating:

$ kubectl get ns/my-namespace
NAME      STATUS         AGE
testing   Terminating    113m

This might be due to a Finalizer in the spec, specific conditions to be met during deletion. You can view the finalizers for your namespace:

...
    "spec": {
       "finalizers": [
           "kubernetes"
       ]
    }
...

This is what a standard namespace with no other conditions might otherwise look like, so if it's hanging, this is likely way.

You can modify the resource using kubectl edit ns/my-namespace, and replace the finalizer with an empty array:

...
    "spec": {
       "finalizers": [
       ]
    }
...

or use the Kube API to update the specification to allow it to finalize manually if this is unsuccessful by dumping the resource:

kubectl get ns/my-namespace -o json > my-namespace.json

and editing that my-namespace.json to remove the finalizer, and then making a PUT request to the API:

kubectl replace --raw "/api/v1/namespaces/my-namespace/finalize" -f ./my-namespace.json

or using an HTTP client to issue the request (usually will require authentication if not exposing via kubectl proxy) using that same json dump:

curl -k -H "Content-Type: application/json" -X PUT --data-binary @my-namespace.json "https://<Kube API Endpoint>/api/v1/namespaces/my-namespace/finalize"

You can then confirm, once you receive a successful response, that the namespace is deleted:

kubectl get ns/my-namespace

which should return no resource.