Make yourself a class that derives from TextBox, and eat the keypress notifications:
Friend Class clsLockedTextBox
Inherits System.Windows.Forms.TextBox
Protected Overrides Sub OnKeyPress(ByVal e As System.Windows.Forms.KeyPressEventArgs)
e.Handled = True
End Sub
End Class
I just tried it, and it works fine. You can't copy text from it with Ctrl-C, but you can code an exception for that.