You could try the text messaging service again. 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 may take a few days.
You can also calling again - 150 from a Virgin landline, or 0345 454 1111, but best to call at 8am to avoid call queues
If you are still in contract there may be some early cancellation fees