Just send a text with a description of cancellation to 07533 051809. Your message will go into a queue and a representative will respond to you as soon as possible. It's not instant and may take a few days.
You can also call 150 from a Virgin landline, or 0345 454 1111, but best to call at 8am to avoid call queues
or write to them to as per the terms and conditions. If writing send it by registered post.