ASP.NET

try

return null;

or get the calling url from the request context and redirect to it

...
routes.MapRoute(
   "DetailOverload",
   "Department/Details",
   new { controller = "Department", action = "VoidDetails" }
);

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

public void VoidDetails()
{
   // do nothing!
} 

public ActionResult Details(int id)
{
  // do something!
}

DepartmentController.cs
{
   public ActionResult Details(int? id)
   {
      if(id == null)
      {
         return View("Home"); // Or "Index", or whatever your base view is called
      }
      // do something
   }
}

Nathan Minier wrote:

Well, the fact is this behavior breaks the expected functionality of HTTP; the appropriate response by convention would be to return an HttpNotFound() if the id is not set or null, or send down a view with a list of possible values.

Generally the expected behavior for HTTP is to send a request and get a response of some sort. Black-holing the response is just not the norm.