have a webservice written in .net which gets the request from the outbound message and sends the response. But salesforce keeps hitting my application and it still waits in outbound queue. How do I stop salesforce to hit the application once the response is received? Following is my code sample

notificationsResponse INotificationBinding.Pur(PTrans.notifications PurP)
    {
        string id,session = null;
        PTrans.notifications test = PurP;
        session = test.SessionId;
        string url = test.EnterpriseUrl;
        PTrans.Transaction__cNotification[] request = PurP.Notification;
        PTrans.notificationsResponse response = new notificationsResponse();
        response.Ack = true;
        return response;
    }
When I tested with soapui, I get the following response,

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <soap:Body>
      <notificationsResponse xmlns="http://soap.sforce.com/2005/09/outbound">
         <Ack>true</Ack>
      </notificationsResponse>
   </soap:Body>
</soap:Envelope>