I found a post regarding Login out of ASP.NET Membership provider from 2010 and the accepted suggestions where
FormsAuthentication.SignOut()
Response.Redirect(FormsAuthentication.LoginUrl)
or
FormsAuthentication.SignOut()
FormsAuthentication.RedirectToLoginPage()
both will create similar results but I would actually suggest to take the Response.Redirect option with the EndResponse flag set to True to terminate processing of the page immediately. otherwise the execution will continue and then you will be redirected.
the original link was http://forums.asp.net/post/2715277.aspx
Enjoy!