title: javascript: if username exists, then not submitting form
body: <p>This is my previous thread about using ajax to check availability of username when registering <br/>
  <a href="https://stackoverflow.com/questions/5675730/jquery-check-username-availability">jQuery: check username availability</a><br />
  Now when I test the system, the ajax works fine, but even I use the existing username, when I click "submit" button, the form will be submitted. I assume I need an additional javascript to stop this from happening, how can I do that? Thanks a lot!</p>