please help, i'm begging

i'm being called daily by Northstar Location Svcs. the first guy who called me, on Nov. 13, said that they received my account from another CA. i negotiated a settlement amount with a CA on this account over 3 years ago, and, being younger and foolish, once it was paid, i got rid of all their info. now i can't remember who it was, all i have is the series of automatic withdrawals from my checking account, and my bank says they have no way of finding out who made the withdrawals. these Northstar people are INCREDIBLY rude, all but calling me a liar, telling me they will call me every day until this balance is paid, etc. i didn't know that i needed to verify with the first CA that they would not sell the balance to someone else. i feel like i'm sinking into quicksand. i told the first Northstar guy, Ron, that i would contact my bank and get back with him. he verified my address and said ok. someone else called me the next day and told me that they received my account directly from Household Bank, which is definitely NOT what Ron told me. on the 19th, a woman called and told me husband that 2 letters had been sent to me since Ron contacted me. we hadn't received anything and told her so. she said they both must have been lost in the mail. yesterday, 11 days after i was first contacted, i received a letter from them. i'm so lost and frustrated, do i have any options???