This is a version of the automatic debit scam as described by the FTC here http://www.ftc.gov/bcp/edu/pubs/consumer/telemarketing/tel06.shtm  It's frequently mysterious as to how they got your account info in the first place. They might be fraudulently charging your account outright or at some time, you might have given/used your account for something in exchange for a $-off coupon,  a free trial, a free shopping card, etc., and buried in the fine print is authorization to auto-charge your account every month with bullsh*t subscriptions that may be very difficult to unsubscribe from. The methods used to get you to "agree" to the fine print is endless and they come up with new schemes every day.
To stop the billing, contact the scammer to kill the subscription and get your money back and if that fails, tell your card issuer or bank that fraud has occurred and have them refund the money and replace the account. Report to the Internet Crime Complaint Center http://www.ic3.gov/complaint/default.aspx , the Federal Communications Commission http://esupport.fcc.gov/complaints.htm , the Federal Trade Commission https://www.ftccomplaintassistant.gov/ and your state AG, as appropriate.