There are unscrupulous people who buy up old debts and old consumer lists, and then they try to intimidate people into paying monies that these people may, in fact, not even owe. If you are quite sure that you never took one out any PayDay loans at all, you are almost certainly right.
To confirm that you are in no trouble with outstanding debts, you can get a free copy of your credit report from each of the three credit reporting agencies courtesy of the Federal Trade Commission by going to https://www.annualcreditreport.com/index.action , and double-check that there's no issue. Annualcreditreport.com is an entirely reputable site and the only one recommended by our government to provide you with a credit report.
If there was a real debt, it is illegal for them to threaten you with jail. So what you would need to do to get these people to stop bothering you is to formally dispute the debt, because you have substantial consumer rights in this area under the Fair Debt Collection Practices Act. Here's a specific section of the FDCPA that you will find useful so that you can dispute any debt and/or send them a cease and desist letter:
sect; 809. Validation of debts [15 USC 1692g]
(a) Within five days after the initial communication with a consumer in connection with the collection of any debt, a debt collector shall, unless the following information is contained in the initial communication or the consumer has paid the debt, send the consumer a written notice containing --(
1) the amount of the debt;
(2) the name of the creditor to whom the debt is owed;
(3) a statement that unless the consumer, within thirty days after receipt of the notice, disputes the validity of the debt, or any portion thereof, the debt will be assumed to be valid by the debt collector;
(4) a statement that if the consumer notifies the debt collector in writing within the thirty-day period that the debt, or any portion thereof, is disputed, the debt collector will obtain verification of the debt or a copy of a judgment against the consumer and a copy of such verification or judgment will be mailed to the consumer by the debt collector; and
(5) a statement that, upon the consumer's written request within the thirty-day period, the debt collector will provide the consumer with the name and address of the original creditor, if different from the current creditor.
(b) If the consumer notifies the debt collector in writing within the thirty-day period described in subsection (a) that the debt, or any portion thereof, is disputed, or that the consumer requests the name and address of the original creditor, the debt collector shall cease collection of the debt, or any disputed portion thereof, until the debt collector obtains verification of the debt or any copy of a judgment, or the name and address of the original creditor, and a copy of such verification or judgment, or name and address of the original creditor, is mailed to the consumer by the debt collector.
(c) The failure of a consumer to dispute the validity of a debt under this section may not be construed by any court as an admission of liability by the consumer.
The above may be hard to understand. However, the Federal Trade Commission gives a wonderful overview of the overaggressive and/or fake collection agencies and how to get them off of your back, where to report them, where to learn more, etc. And it is much more straightforward than the law. You can access that here.
But basically, you want to dispute the debt -- that's the term you use with them, and you can do it in writing if you have their address. Otherwise just tell them that you want written proof you owe this debt, in writing and whether they say they will give it to you or not, let them know that you are disputing the debt as provided for by the Fair Debt Collection Practices Act and that they should cease and desist from any further contact with you. Once you have disputed the debt, they are required to leave you alone. If they don't, you can report every harassing phone call these folks make to the Federal Trade Commission at FTC.gov, and they can be fined $1,000 for each incident.
If these folks are out-and-out scammers, they may not be deterred by your invoking the name of the Federal Trade Commission or the Fair Debt Collection Practices Act, but they will also know that you know your rights and that you're not about to pay up if they can't prove you owe this debt. Once you tell them you're disputing the debt you can just ignore them. If they are legitimate and the debt is legitimate, they will have to take you to court. But I don't; think that's ever going to happen here if you're sure you never took out any Payday loans.