default.aspx

 <script runat="server"> 
   //This snippet goes into sitecore/login/default.aspx
    protected override void OnPreInit(EventArgs e)
    {
      if (Request.Cookies[".AspNet.Cookies"] != null)
       {
           //Force expiration of SSO cookie on .yourclient.com domain
           HttpCookie myCookie = new HttpCookie(".AspNet.Cookies");
           myCookie.Expires = DateTime.Now.AddDays(-1d);
           myCookie.Domain = "yourclient.com"; // !!!!
           Response.Cookies.Add(myCookie);
       }
     }
</script>