Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

procedure TForm1.Memo1Change(Sender: TObject);
 const
   MaxLineCount = 5;
 begin
   if Memo1.Lines.Count > MaxLineCount then
     // undo the last change 
    // letze Дnderung rьckgдngig machen 
    Memo1.Perform(EM_UNDO, 0, 0);
   // The EM_EMPTYUNDOBUFFER message clears the undo flag, 
  // which means that you can no longer undo your last change 
  // to the edit control. 
  // Die Message EM_EMPTYUNDOBUFFER lцscht das UnDo Flag, 
  // damit kann die letzte Дnderung nicht Rьckgдngig gemacht werden. 
  Memo1.Perform(EM_EMPTYUNDOBUFFER, 0, 0);
 end;