If you are using forms authentication then you can use FormsAuthentication.SignOut();
this will work as long as you have set up ur website for form authentication in the config like this:
<authentication mode="Forms">
<forms loginUrl="~/login.aspx" slidingExpiration="false"
cookieless="AutoDetect" timeout="20" protection="All"
enableCrossAppRedirects="false"
defaultUrl="~/authenticated.aspx"></forms>
</authentication>
If you using some other mechanism e.g. some session based thing then you do this
Session.Abandon(); and then you can Response.Redirect("~/logn.aspx") i.e to you login page
More information is available on MSDN:
http://msdn.microsoft.com/en-us/library/system.web.security.formsauthentication.signout.aspx
http://msdn.microsoft.com/en-us/library/system.web.security.formsauthentication.redirecttologinpage.aspx