Asp.Net web service: I would like to return error 403 forbidden

I have got a web service programmed in c# / asp.net.

[WebService(Namespace = "http://example.com/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[ScriptService]
[System.ComponentModel.ToolboxItem(false)]
public class Service: System.Web.Services.WebService
{

    [WebMethod]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public Result GetData()
    {
        User user = GetUser();

        if (user.LoggedIn)
        {
            return GetData();
        }
        else
        {
            // raise exception -> return error 403
        }
    }

How is it possible to return error 403 out of this web service? I can throw an exception - but this shows the exeption and not his error.

Any ideas?