Did you send it to them via certified US mail? If not, they will claim they never got it of course! Also send them a cease & desist letter (you can find a free template online) via certified US mail. Send them a certified copy of your card, not the real one. You can take it down and have it notarized, then send it via a certified US mail. That way they can't say they didn't get it.
Read up on your rights:
http://www.ftc.gov/os/statutes/fdcpajump.shtm
http://www.consumerfinance.gov/askcfpb/search ...
http://www.consumer.ftc.gov/articles/pdf-0096-fair-credit-reporting-act.pdf
Report them here also:
File a complaint with your states Attorney General
https://www.ftccomplaintassistant.gov/#&panel1-1
https://esupport.fcc.gov/ccmsforms/form1088.action
https://complaints.donotcall.gov/complaint/complaintcheck.aspx?panel=2
http://www.consumerfinance.gov/complaint/