Handling browser back button after logout

This happens because of Cache.

Clear the cache such that browser has no history (this will make back/forward button in browser grayed out disabled.)
The code for clearing cache can be put up in code behind as follows:

// Code disables caching by browser. Hence the back browser button
// grayed out and could not causes the Page_Load event to fire 
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetExpires(DateTime.UtcNow.AddHours(-1));
Response.Cache.SetNoStore();

You can add something similar in form aspx if you want to place it there:

<META Http-Equiv="Cache-Control" Content="no-cache">
<META Http-Equiv="Pragma" Content="no-cache">
<META Http-Equiv="Expires" Content="0">

Just feels like I answered you this last week, hope it's not a re-post and this solves the problem for you.