This same thing happened to me, I bought a Direct TV Tivo box off ebay for $20 and DTV said they would not send me a card and wanted $100 to lease me their own DVR instead. I said send me a card or I am going to Comcast and they sent the card. But I had to agree to a 2 year contract and pay $5 a month for a second DVR, which is what they used to charge anyway before they started calling it a lease.
You need to go to the loss prevention department, just call up and say you want to cancel, they will ask what can they do to keep you, tell them to activate your box and send a card. If you are under a contract and can't cancel you might just have to keep trying until you get a nice rep on the line that is willing to help you out.