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

By default, once a user has signed into OpenShift, they are allowed to create new projects. This can be disabled in the following ways:

• Add projectRequestMessage to project.config.openshift.io/cluster
• Set clusterrolebindings.rbac self-provisioners subjects to null

Edit the project.config.openshift.io/cluster resource.

oc edit project.config.openshift.io/cluster

And add projectRequestMessage. This might be something you want to do in a scenario where you have some other method for creating projects, such as with an orchestration technology such as Terraform.

spec:
  projectRequestMessage: New project creation has been disabled

The pods in the openshift-apiserver namespace should automatically restart. After each pod has restarted, the change should be live.

~]$ oc get pods --namespace openshift-apiserver
NAME                         READY   STATUS        RESTARTS   AGE
apiserver-59c8df77f6-qpzmx   0/2     Pending       0          41s
apiserver-846d775c67-kjjdx   2/2     Running       0          44d
apiserver-846d775c67-r8n5c   2/2     Terminating   0          44d
apiserver-846d775c67-wj665   2/2     Running       0          44d

Let use the oc new-project to try to create a new project.

oc new-project my-project

And something like this should be returned.

~]$ oc new-project my-project
Error from server (Forbidden): New project creation has been disabled