CancellingCADImport.cs

        public Result OnStartup(UIControlledApplication a)
        {
            a.ControlledApplication.FileImporting += ControlledApplicationOnFileImporting;
            return Result.Succeeded;
        }

        private void ControlledApplicationOnFileImporting(object sender, FileImportingEventArgs e)
        {
            TaskDialog.Show("Alert", "Sorry about that. No CAD import for you.");
            e.Cancel();
        }