How do i introduce validation before remote_form_for submits?

I have javascript function called validateForm(). I am able to call it before the AJAX request process. I tried to use return false and event.preventDefault. But there seems to be no effect. Here is what my code looks like

<% remote_form_for :customer, :update =>"uxScreenLoaderDiv", :url => {:action => "login", :controller => "site"}, :html => {:id => "uxLoginForm"}, :onsubmit => "validateForm(event)" do |f| %>
    User name : <%= f.text_field "uxUserName", :class => "TextBox", :style => "width:100px;" %>&nbsp;*
    &nbsp;Password : <%= f.password_field "uxPassword", :class => "TextBox", :style => "width:100px;" %>&nbsp;*
    <%= f.submit "Go!", :class => "Button-Simple", :id => "uxSubmitButton" %>   
<% end %>

the javascript function is simple as follows

function validateForm(event){
   return false;
   //event.preventDefault();
}