Steps to reproduce: 

1. Handle the MessageShowing event like this: 

private void SpreadsheetElement_MessageShowing(object sender, Telerik.WinForms.Controls.Spreadsheet.MessageShowingEventArgs e)
{
    e.IsHandled = true;
}

2. Protect the Workbook 

3. Start the app and paste with the context menu

Actual: A dialog is shown