You could add to the modules:
Code:
    // disable only when entire cart is free shipping
    if (zen_get_shipping_enabled($this->code)) {
      $this->enabled = ((MODULE_SHIPPING_PERWEIGHTUNIT_STATUS == 'True') ? true : false);
    }

// bof: turn off for weight over 2
      if (!IS_ADMIN_FLAG) {
        global $cart;
        if ($_SESSION['cart']->show_weight() > 2) {
          $this->enabled = false;
        }
      }
// eof: turn off for weight over 2