Remove cookie on log-out

You can try with

Session.Abandon();
Response.Cookies.Clear();

Or also

YourCookies.Expires = DateTime.Now.AddDays(-1d);

Link : http://msdn.microsoft.com/en-us/library/ms178195%28v=vs.100%29.aspx

From MSDN DOCUMENTATION,

You cannot directly delete a cookie on a user's computer. However, you can direct the user's browser to delete the cookie by setting the cookie's expiration date to a past date. The next time a user makes a request to a page within the domain or path that set the cookie, the browser will determine that the cookie has expired and remove it.

You can do like this:

if (Request.Cookies["Administrator"] != null)
{
    HttpCookie myCookie = new HttpCookie("Administrator");
    myCookie.Expires = DateTime.Now.AddDays(-1d);
    Response.Cookies.Add(myCookie);
}