dental cost containment plan

I signed up July 1 for the dental cost containment plan, with coverage due to begin August 1. I changed my mind in the meantime, before Aug. 1, but they refused to refund my money. I then told them to cancel my coverage (before it had gone into effect), and they said I could cancel, but would still not get a refund. But apparently this was a lie, since the promised letter confirming my cancellation never came....