登录时如何记住账号，关于cookie的使用

checkbox打钩就记住账号，下次登陆时如果上次是记住的则在输入账号的地方自动天上账号，并且checkbox也已经钩上
如果开始已经记住了cookie，现在把钩去掉，那怎么清除这个cookie

记录COOKIE
HttpCookie username = new HttpCookie("username");
HttpCookie autolog = new HttpCookie("autologon");
username.Value = TextBox1.Text;
autolog.Value = CheckBox1.Checked.ToString();
//保存一年
username.Expires = DateTime.Today.AddYears(1);
autolog.Expires = DateTime.Today.AddYears(1);
Response.Cookies.Add(username);
Response.Cookies.Add(autolog);

在FORM_LOAD中读取COOKIE
if (Request.Cookies["username"]!= null)
{
TextBox1.Text = Request.Cookies["username"].Value;
CheckBox1.Checked = bool.Parse(Request.Cookies["autologon"].Value);
}

记住：
if(checkBox1.Checked){
HttpCookie cookie = new HttpCookie("username", "张三");
cookie.Expires = DateTime.Now.AddYears(1);
Response.Cookies.Add(cookie);
}