kyverno/samples/best_practices/disallow_default_namespace.yaml

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: disallow-default-namespace
  annotations:
    policies.kyverno.io/category: Workload Isolation
    policies.kyverno.io/description: Kubernetes namespaces are an optional feature 
      that provide a way to segment and isolate cluster resources across multiple 
      applications and users. As a best practice, workloads should be isolated with 
      namespaces. Namespaces should be required and the default (empty) namespace 
      should not be used.
spec:
  rules:
  - name: validate-namespace
    match:
      resources:
        kinds:
        - Pod
    validate:
      message: "Using 'default' namespace is not allowed"
      pattern:
        metadata:
          namespace: "!default"
  - name: require-namespace
    match:
      resources:
        kinds:
        - Pod
    validate:
      message: "A namespace is required"
      pattern:
        metadata:
          namespace: "?*"