You can't prevent the back key working in the browser - it pulls pages from it's internal cache so it will return to login pages.
However, you can set the Login page to uncacheable in the Page_Load event:
Response.Cache.SetExpires(DateTime.UtcNow.AddMinutes(-1));
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetNoStore();
and then check if your user is logged in. If he is, redirect him to your Home page (or another suitable one)