var document = webBrowser1.Document;
document.ExecCommand("ClearAuthenticationCache", false, null);
找了好几天才找着，国内搜索已经被百毒打败。
Delete cookies in webBrowser without restarthttp://stackoverflow.com/questions/21265674/delete-cookies-in-webbrowser-without-restart
原文：http://www.cnblogs.com/gezifeiyang/p/5983984.html