Session signout query

I'm using forms authentication on my site, and on my 'signout' button I use:
FormsAuthentication.SignOut();
Response.Redirect("LogIn.aspx");
which takes me back to the login page - which is fine. However, I was under the impression that using theSignout() method would make the program force you to re-signin, but all I have to do is press the 'back' button on the browser and I'm back in the site again??? And clicking on any of the other links allows me to still freely navigate around the site. How can I make this more secure?
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