Unfortunately some people do not know how to properly communicate. You may need to take extra steps to prevent overspray and loose balls.

It may be a scam. Whenever one is unsure, give no information, demand proof in writing (do not give your address or other information),...

If it was settled, and that can be proven, they are not entitled to payment.

You are probably uncollectible. When the learn this they might stop collections. If not, once you are no longer uncollectible, bankruptcy might be...

You can attempt to do so by a letter.

You may have to file a lawsuit against them if they refuse to stop pursuing this debt.

It may be a scam. Whenever one is unsure, give no information, demand proof in writing (do not give your address or other information),...

The statute of limitations is the deadline to file the lawsuit.

You should put up your best defense and negotiate a settlement. Note, You must respond in the legal way and on time with, for example a motion...

You may have a case to report to the FBI.