1) You could try the text messaging service.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. VM are trying to answer queries within 24 hours, however it may take longer on a weekend, but it may take a few days.
2) You can also call 150 from a Virgin landline, or 0345 454 1111, but best to call at 8am to avoid call queues
3) Write to them to as per the terms and conditions. If writing send it by registered post.