How can i cancel the AsynsTask in Android?

Hi onCancel of dialog i want to cancel to server call but
i m facing problem that even i cancel the task, it hits my server and modifies the data.
How can I resolve this issue ? Below is my
code..

            private class UserBoardingTask extends AsyncTask {
 @Override

            protected void onPreExecute() {
 progressDialog = new
            ProgressDialog(getActivity());

            progressDialog.setMessage(getResources().getString(R.string.please_wait));

            progressDialog.setIndeterminate(false);

            progressDialog.setCancelable(true);
 progressDialog.setOnCancelListener(new
            OnCancelListener() {
 @Override

 public void
            onCancel(DialogInterface dialog) {
 if (userOnBoardingTask!= null &&
            userOnBoardingTask.getStatus() != AsyncTask.Status.FINISHED &&
            !userOnBoardingTask.isCancelled()) {

            userOnBoardingTask.cancel(true);

 } 
 }

            });
 progressDialog.show();
 }



            @Override
 protected Void doInBackground(String... urls) {
 String
            boardingURL=null;


 boardingURL=
            getUrl();


 UserOnBoardingDTO userOnBoardingDetailsDTO =
            AppStateManager.getUserBoardingDetails();

            try{


 RestAPIManager.putToNSWebService(boardingURL,
            userOnBoardingDetailsDTO, username, password);

 }
            


 catch (Exception e) {

            errorMessage=getResources().getString(R.string.unknown_exp);
 }

            return null;

 }

 @Override

            protected void onCancelled() {
 super.onCancelled();

            closeProgressDialog();
 errorMessage="";

            AppStateManager.setUserBoardingDetails(null);

            userOnBoardingTask=null;


 }


            @Override
 protected void onPostExecute(Void res) {

            closeProgressDialog();
 userOnBoardingTask=null;

            if(!FieldsValidator.isBlank(errorMessage)){

            CommonUtil.showToast(getActivity(),errorMessage);
 errorMessage="";

            return; 

 }