It needs a few more things to kill in order to sign the user out... check out this sample method:

public static void SignoutUser(System.Web.HttpContext httpContext, DotNetNuke.Entities.Portals.PortalSettings ps)
  {
   // sign them out

   // Log User Off from Cookie Authentication System
   System.Web.Security.FormsAuthentication.SignOut();

   //remove language cookie
   httpContext.Response.Cookies["language"].Value = "";

   // expire cookies
   if (DotNetNuke.Security.PortalSecurity.IsInRoles(ps.AdministratorRoleId.ToString()) && httpContext.Request.Cookies["_Tab_Admin_Content" + ps.PortalId.ToString()] != null)
   {
    httpContext.Response.Cookies["_Tab_Admin_Content" + ps.PortalId.ToString()].Value = null;
    httpContext.Response.Cookies["_Tab_Admin_Content" + ps.PortalId.ToString()].Path = "/";
    httpContext.Response.Cookies["_Tab_Admin_Content" + ps.PortalId.ToString()].Expires = DateTime.Now.AddYears(-30);
   }

   httpContext.Response.Cookies["portalaliasid"].Value = null;
   httpContext.Response.Cookies["portalaliasid"].Path = "/";
   httpContext.Response.Cookies["portalaliasid"].Expires = DateTime.Now.AddYears(-30);

   httpContext.Response.Cookies["portalroles"].Value = null;
   httpContext.Response.Cookies["portalroles"].Path = "/";
   httpContext.Response.Cookies["portalroles"].Expires = DateTime.Now.AddYears(-30);

   if (httpContext.Request.Cookies["_Tab_Admin_Preview" + ps.PortalId.ToString()] != null)
   {
    httpContext.Response.Cookies["_Tab_Admin_Preview" + ps.PortalId.ToString()].Value = "False";
   }
  }

I know this is an old post, but I wanted to document wherever this code exists that there is a better way of clearing the user's roles cookie, with is with the PortalRoles.ClearRoles() shared/static method. That encapsulates the three lines that otherwise modify the portalroles cookie above.

Using the API is always a better way to work with the system if possible because it allows for changes to be made to the internal workings. I'm currently working on a resolution for the limit on the number of roles in which a user exists prior to the system falling over because of truncation of data (causing a decryption exception) in the user's roles cookie.

Whilst the code here won't affect what I'm doing, I thought I'd post the API method of doing the same thing. I'm just trying to do my bit for interoperability throughout the community!

To summarise,

<code>
PortalSecurity.ClearRoles()
</code>

replaces

<code>
Response.Cookies["portalroles"].Value = Nothing;
Response.Cookies["portalroles"].Path = "/";
Response.Cookies["portalroles"].Expires = DateTime.Now.AddYears(-30);
</code>