How to stop the function when its finished its task

Hi,
I have developed to check the username availability in registration form. It is functioning very well
when you click the "Check availability" button. But when you click the submit button with the username that is already existing in the db, it is saying the username is already existing but it is inserting the records into db. I don't want to insert the records when the username is already existing.

Here is the code in head tag
----------------------------------------------
<script type="text/javascript">
       var emptyUserNameMessage = 'Please enter the username';
       var progressUserNameMessage = 'Checking...';
       var availableUserNameMessage = 'Username is available';
       var usedUserNameMessage = 'Username has been taken';


       $(function () {
           var userNameAvailabilityLabel = $('#<%= UserNameAvailabilityLabel.ClientID %>');

           $('#<%= UserNameAvailabilityButton.ClientID %>').click(function () {
               var userNameTextBox = $('#<%= UserNameTextBox.ClientID %>');
               var userName = userNameTextBox.val();
               if ($.trim(userName) == '') {
                   userNameAvailabilityLabel
                            .removeClass()
                            .addClass('required')
                            .html(emptyUserNameMessage);
               }
               else {
                   userNameAvailabilityLabel.html('');
                   $('#ProgressDiv').show();

                   $.ajax({
                       type: 'POST',
                       url: 'Sample.asmx/CheckUserNameAvailability',
                       data: '{userName: \'' + userNameTextBox.val() + '\'}',
                       contentType: 'application/json; charset=utf-8',
                       dataType: 'json',
                       success: OnCheckUserNameAvailabilitySuccess,
                       error: OnCheckUserNameAvailabilityError
                   });
               }
               return false; //Prevent postback
           });

           function OnCheckUserNameAvailabilitySuccess(response) {
               $('#ProgressDiv').hide();
               if (response != null && response.d != null) {
                   var data = response.d;
                   switch (data) {
                       case 0:
                           userNameAvailabilityLabel
                            .removeClass()
                            .addClass('available')
                            .html(availableUserNameMessage);

                           break;
                       case 1:
                           userNameAvailabilityLabel
                            .removeClass()
                            .addClass('used')
                            .html(usedUserNameMessage);
                           break;
                   }
               }
           }
           function OnCheckUserNameAvailabilityError(xhr, ajaxOptions, thrownError) {
               alert(xhr.statusText);
           }
       });
 
    </script>

Here is the code in code behind file
---------------------------------------------------
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class Registration : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void OnCheckUserNameAvailabilitySuccess()
    {
        Response.Write("Username is already taken");
    }
    protected void SubmitButton_Click(object sender, EventArgs e)
    {

        Customers com = new Customers();
        int CustomerID=0;
        string Username = UserNameTextBox.Text;
        string passwords = PasswordTextBox.Text;
        string repasswords = RetypePasswordTextBox.Text;
        string firstname = FirstnameTextBox.Text;
        string lastname = LastnameTextBox.Text;
        string Address1 = Address1TextBox.Text;
        string Address2 = Address2TextBox.Text;
        string PostalCode = PostalCodeTextBox.Text;
        string Customermail = EmailTextBox.Text;
        string Phone = ContactNoTextBox.Text;

           

        if (IsPostBack!=null)
        {

                OnCheckUserNameAvailabilitySuccess();
           
                Customers.InsertCustomers(CustomerID, Username, passwords, firstname, lastname, Address1, Address2, PostalCode, Customermail, Phone);

                Response.Write("You have registered successfully");
                UserNameTextBox.Text = "";
                PasswordTextBox.Text = "";
                RetypePasswordTextBox.Text = "";
                FirstnameTextBox.Text = "";
                LastnameTextBox.Text = "";
                Address1TextBox.Text = "";
                Address2TextBox.Text = "";
                PostalCodeTextBox.Text = "";
                EmailTextBox.Text = "";
                ContactNoTextBox.Text = "";
           
           




        }
        else
        {

            Response.StatusCode = 404;
            this.Visible = false;
            Response.Write("404 Page Not Found");


        }
    }

   


}