Disable Submit Button on Validation

I have a form the has a textbox that validates if a username already exists in the database. If a user exists in the database a message appears next to the textbox. I want it when a username exists in the database for the message to appear and for the submit button to be disabled. How can I accomplish this? Below is my code for the aspx page, vb code and js code:
.vb Code
Imports System Imports System.Web Imports System.Web.UI Imports System.Web.UI.WebControls Namespace myControls ''' <summary> ''' Enables you to perform custom validation on both the client and server ''' </summary> Public Class AjaxValidator Inherits BaseValidator Implements ICallbackEventHandler Public Event ServerValidate As ServerValidateEventHandler Dim _controlToValidateValue As String Protected Overrides Sub OnPreRe ...