Hi and welcome to the forums, belleRiz. 👋

To disable the PM; go to your profile then click on 'Customize Profile' then click on 'Edit Options' then from there go into the 'Messaging & Notifications' and there you will see a section which says 'Private Messaging' - uncheck the box that says 'Enable Private Messaging'. Finally, scroll to the bottom and click the button that says 'Save Changes.' That's it.

Welcome.