How can I fight against "national readers service" scam?

National readers service, about half year ago,called me over and over and said I had signed a membership about some magazine subscriptions.And they pushed me over and over and finally got my credit card number. But when I asked them to show me the documents I signed,they declined to do so.When I called them back to cancel the membership, they told me it can not be cancelled.Then I searched the internet, and found so many complaints about this company’s scam and fraud.Of course,when I found they charged my credit card, I disputed it. And I never receive any magazine from this company. Yesterday I received a letter from Credit & Collection Solutions. They said I owed National readers service about $1000 and I need pay about $500 to close my account. Otherwise they willsend me to the court.