How do I stop a node submission programmatically? i'm using Drupal 8. I would like to stop the user to submit a node when he tap save, and show some message. How can I do it programmatically?

Thanks!
You can hide the form submit button using hook_form_alter() and show a link Save and Publish on form. And include a jQuery snippet to show message, which will be triggered when user clicks on a link Save and Publish.