Cashnet USA Scamming me twice nationwide Nationwide

 I took a loan out with Cashnetusa in 2007, the money was being paid back in installments from my bank account. The loan was for $250 and I paid them $400 before I moved and changed my account. The end of this past year 2013 I was contacted by someone saying that they represented cashnet and that I owe them $560 for unpaid loan and they threatened to put me in jail for fraud. Not remembering if I owed them or not and not wanting to go to jail I paid them. Now this past week I'm getting threatening calls again saying that I owe them $600. I contacted the person that left the message and explained to them that I just paid this debt. They asked me for the loan account number, and when I gave it to them they told me this is a different account number. I got in touch with the person that I paid a few months ago and he told me this new number is a scam. Now I believe that either they both are scammers, or cashnet sold the account to two collection agencies. Either way I don't want to pay again, what can I do to stop this once and for all? They are going to post to my credit report.