They keep reselling a debt I paid off long ago

Hi! A year ago I was contacted by an attorney's office saying they are trying to collect a debt from me, original creditor being Maryland National Bank (MBNA). This was a credit card balance I had paid off long ago and I had never even been late with the payments. In fact, on my 3 credit reports, one says the account is in good standing, one says I owe them $10 but it has not gone to collections, and only Experian says that I owe them thousands of $$, which is what they claim. I answered with a c&d letter and never heard from them again. In April 08 I received an identical letter from another lawyer's office, I did the same, never heard from them. Two days ago I received a similar letter from Penta Group and I am wondering how long will this be going on? I keep asking for validation but they never provide it, they just sell the alleged "debt". What can I do? In the meantime it is affecting my credit report. I cannot even invoke the SOL because I don't know in which state they claim I had the debt! How can I get them to send me validation? And how can I prove I paid it off? I don't keep statements from 2002 for all my credit cards (I will from now on, though!) Thank you for any help!