If you're asking how you can undo the early activation of your SIM, I don't think you can. But as you will probably have the SIM within a day or two anyway (they are sent first class) and Smarty's plans are so cheap, you will probably miss out on less than a pound's worth of service anyway, so it's not really a big deal.
If you no longer want the Smarty service at all, chat to support and they can arrange to cancel it and refund you.
