AutoCAD .NET :: How To Stop Changing Active Document

How can I stop changing active document? I was thinking I could use this, but I don't know how to cancel it in the event handler:
 
AcadApp.DocumentManager.DocumentToBeActivated += new DocumentCollectionEventHandler(DocumentManager_DocumentToBeActivated);static void DocumentManager_DocumentToBeActivated(object sender, DocumentCollectionEventArgs e){ // Messagebox to ask if user are sure bool AreYouSureYouWantToChangeActiveFile = MessageBox.... if(!AreYouSureYouWantToChangeActiveFile) //cancel change active file somehow...???}