If you are not familiar with the oc command, refer to OpenShift - Getting Started with the oc command.

Before you can delete the built in kubeadmin user account, you will need to assign the cluster-admin Cluster Role Binding to some other user account. 

~]$ oc adm policy add-cluster-role-to-user cluster-admin john.doe
clusterrole.rbac.authorization.k8s.io/cluster-admin added: "john.doe"

In the kube-system namespace, there should be a secret named kubeadmin.

~]$ oc get secret kubeadmin --namespace kube-system --output yaml
apiVersion: v1
data:
  kubeadmin: JDJhJDEwJDJiUExscmNDd1RDamk4QUs1ak5oOGUzblp1RU1XSndCZFkuQUZ0MmdXTEMxUGRndER3d1FD
kind: Secret
metadata:
  creationTimestamp: "2021-07-16T17:09:50Z"
  name: kubeadmin
  namespace: kube-system
  resourceVersion: "90"
  uid: f86b63f7-dfe4-423f-9838-e70af0e0dafe
type: Opaque

Deleting the kubeadmin secret will effectively delete the built in kubeadmin user account.

oc delete secret kubeadmin --namespace kube-system