Remove the cookie from the System.Web.UI.Page.Request.Cookies collection.
Remove the cookie from the System.Web.UI.Page.Request.Browser.Cookies collection.
Set the Expires property to DataTime.Now for a cookie in the Web.UI.Page.Response.Cookies
Remove the cookie from the System.Web.UI.Page.Response.Cookies collection.
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. Useful resources, How to: Delete a Cookie