It's not a very beautiful thing, but I think it may work:
On logout redirect the user to your home page, adding just a little $_GET variable. Redirect Link should be
Insert, into this page or your Joomla Template index.php, a script that will prevent the back button from functioning whenn the variable $_GET['logged_out'] is set. This part is really tricky, because no method to my knowledge will actually work in every situation. The easiest way to disable the back button is IMO to actually fool it:
This will redirect the user to your home page another time. As I said, it is not very beautiful, but I doubt that there are beautiful ways to do at all. In fact, I would strongly suggest to evaluate if this is truly necessary.