Remove cookie.

To remove cookie from server one need to set its expiry time to current time.

e.g. J2EE container one can remove the cookie in following way

HttpServletResponse response = (HttpServletResponse) servletResponse;
Cookie cookie = new Cookie("cookie-key", "");
.... set all the other attributes same as that of existing cookie, like path and domain.  
cookie.setMaxAge(0);
response.addCookie(cookie);

setting max age of cookie to '0' I am setting its expiry to current machine time.

JavaScript

document.cookie = "username=; expires=Thu, 01 Jan 1970 00:00:00 GMT";

For more details refer