' zunächst die benötigten API-Deklarationen
Private Declare Function SendMessage Lib "user32" Alias _
  "SendMessageA" (ByVal hWnd As Long, ByVal wMsg As _
  Long, ByVal wParam As Long, lParam As Any) As Long
 
Private Const EM_LINEFROMCHAR = &HC9
Private Const EM_LINEINDEX = &HBB
Private Const EM_GETLINECOUNT = &HBA

' Zeilennummer der aktuellen Eingabeposition ermitteln
Public Function GetCurrentLine(hWnd As Long) As Long
  GetCurrentLine = SendMessage(hWnd, EM_LINEFROMCHAR, _
    -1, ByVal 0&) + 1
End Function

' Anzahl der Zeilen der TextBox ermitteln
Private Function GetLineCount(ByVal hWnd As Long) As Long
  GetLineCount = SendMessage(hWnd, EM_GETLINECOUNT, -1, 0&)
End Function

Private Sub RichTextBox1_KeyDown(KeyCode As Integer, Shift As Integer)
  With RichTextBox1
    Select Case KeyCode
      Case vbKeyUp
        If GetCurrentLine(.hWnd) < 2 Then
          KeyCode = 0
        End If
 
      Case vbKeyDown
        If GetCurrentLine(.hWnd) = GetLineCount(.hWnd) Then
          KeyCode = 0
        End If
    End Select
  End With
End Sub

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