This being an obvious approach to how Dashboards are by default seen by all. And how you can disable/hide for users. Even for D365 System Administrators!
Unlike usual forms, view or other records,
And Deleting the Dashboard isn’t an option.
Enable Security Roles way
So a simple trick is to remove entire access from Enable Security Roles
Make sure Display only to these selected security roles
And then just untick if you want to provide no access to anyone
Hope this works!