Private Sub Form1_KeyPress(ByVal sender As Object, ByVal e As _
  System.Windows.Forms.KeyPressEventArgs) Handles Me.KeyPress
        Select Case Asc(e.KeyChar)
            Case 97, 115, 8            ' a,s, und Backspace zulassen
            Case Else
                e.Handled = True       ' alle anderen Eingaben unterdrücken
        End Select
End Sub