How to prevent an asp.net mvc controller from being instantiated and executed

If you want to change controllers dynamically, the place to do it is in a custom RouteBase subclass. Redirection is a bad idea because your server will issue a 301 or 302 to the client's browser telling it to lookup a new location on the server. That round-trip is wasteful and unnecessary.

public class SwitcherRoute : RouteBase
{
    public override RouteData GetRouteData(HttpContextBase httpContext)
    {
        RouteData result = null;

        // Trim the leading slash
        var path = httpContext.Request.Path.Substring(1);

        // Check that the request is what you are interested in
        if (path.Equals("home/about", StringComparison.OrdinalIgnoreCase))
        {
            result = new RouteData(this, new MvcRouteHandler());


            if (/* some (preferably cached) condition */)
            {
                result.Values["controller"] = "Home";
                result.Values["action"] = "About";
            } 
            else 
            {
                result.Values["controller"] = "Alternate";
                result.Values["action"] = "About";
            }
        }

        // IMPORTANT: Always return null if there is no match.
        // This tells .NET routing to check the next route that is registered.
        return result;
    }

    public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
    {
        VirtualPathData result = null;

        var controller = Convert.ToString(values["controller"]);
        var action = Convert.ToString(values["action"]);

        if (action.Equals("About", StringComparison.OrdinalIgnoreCase))
        {
            if (controller.Equals("Home", StringComparison.OrdinalIgnoreCase) || 
                controller.Equals("Alternate", StringComparison.OrdinalIgnoreCase))
            {
                return new VirtualPathData(this, "home/about");
            }
        }

        // IMPORTANT: Always return null if there is no match.
        // This tells .NET routing to check the next route that is registered.
        return result;
    }
}

Usage

routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

routes.Add(
    name: "AboutPage", 
    item: new SwitcherRoute());

routes.MapRoute(
    name: "Default",
    url: "{controller}/{action}/{id}",
    defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);

Note that you can make your custom route more flexible by accepting constructor parameters or even inheriting from the built-in Route class.

Also note that routes run on every request, so you should cache any expensive lookup operations as in this example.