How do I remove dashboard access from specific user roles?

To lock subscribers and contributors out of the admin:

function wpse23007_redirect(){
  if( is_admin() && !defined('DOING_AJAX') && ( current_user_can('subscriber') || current_user_can('contributor') ) ){
    wp_redirect(home_url());
    exit;
  }
}
add_action('init','wpse23007_redirect');

Hope that helps. All roles give the user a capability that is the name of that role, so you can use any role name as a capability.