How can I be certain that a junk debt buyer of my bankrupt account will leave me alone?

I recently got a letter from a collection agency (NCO Financial)in the mail stating that I owe $11,000 for a past due Chase credit card. I called them and let them know that I never owned a Chase credit card. I got all 3 of my credit reports back in 2003 and there was nothing on them showing a past due amount. So I called and the person at the other end stated that the card was opened in 1990 and asked if I filed bankruptcy then - which I had. I asked him if I can ignore the letter and he said yes. However, something didn't seem on the up and up. Just a gut feeling.