You can try this
[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
{
Context.Response.Status = "403 Forbidden";
//the next line is untested - thanks to strider for this line
Context.Response.StatusCode = 403;
Context.Response.End();
return null;
}
}